You're right to think that Dante wasn't alone. There were also Giovanni Boccaccio, author of the Decameron, a collection of 100 short stories, and Francesco Petrarca, known as Petrarch, author of the 366 poems that make up his Canzoniere. These two were good friends and even read each other's drafts, and they were alive at the same time as Dante, but much younger. Together, they are known as the Tre Corone, the Three Crowns of Italian literature, and all three were of Florentine extraction. Petrarch was actually far more influential than Dante, as his limited, refined lexicon was seen as much more beautiful. In fact, "petrarchism" would be the dominant poetic style of the Renaissance for the next several centuries, until Giambattista Marino and his imitators ushered in the Baroque.

The impact of these three authors was solidified by the work of the humanist Pietro Bembo, whose Prose della volgar lingua (1525) effectively argued that Petrarch's language was the most beautiful manifestation of the dialects of Italy. The Academia della crusca, founded in 1583, was a society of humanists and poets who took it upon themselves to become the arbiters of the Italian language. In 1612, they published the very first dictionary of Italian, based on the conception that a certain version of Tuscan dialect was the ideal literary language. It's important to note that this was not the dialect of the streets, but a refined, almost artificial language, composed largely of tuscanized Latin and latinized Tuscan. Unsurprisingly, the vast majority of their vocabulary is drawn from Petrarch and Boccaccio, but also other influential Florentines like Machiavelli, Tasso, Guicciardini, Castiglione, and Galileo.

More can doubtless be written, but while waiting, you might like to see what /u/AlviseFalier has written (extensively). I'll point to "If Italy didn’t exist as a unified state until 1861, and each region had its own language, what exactly is the Italian language then? Is it an amalgamation of every regions languages? Or was Italian spoken in one part of the peninsula and then spread and declared the official language?" . I point to it because, while they don't provide the answer there, they point to two previous postings and contextualize them.

This is not to discourage discussion. Further questions, data, and debate are always welcome.

Hi there. I think I can shed some more light on your specific questions: while the Tuscans most certainly weren't the only ones writing in their local vernacular, the enormous, lasting, and unquestioned popularity of Tuscan literature first turned knowledge of tuscan into a sort of unofficial shibboleth amongst Italian intellectuals, and eventually became the most convenient way for Italians with some degree of formal schooling to communicate amongst each other. Indeed, the body of medieval tuscan literature is vast: Dante Alighieri not only wrote the Divine Comedy, but left a number of writings behind, including the De Vulgari Eloquentia, a latin treatise on the literary value of his local dialect. The great tradition of 14th century Tuscan poets and writers also includes, among others, poets like Giovanni Boccaccio and Francesco Petrarca (better known in english as Petrarch).

But there's certainly a long history of documents written in the vernaculars from all over Italy, ranging from legal documents written appearing near tenth-century Benevento, to fifteenth-century fencing manuals written in Venetian, even though the overwhelming prevalence of Latin for use in official, legal, and ecclesiastic contexts dimmed the need for an new universally understood language until relatively late; in spite of the popularity of tuscan literature, as late as 16th century there remained an fervent intellectual camp in favor of classical latin as the Miglior Lingua, or "Better Language," in spite its distance between everyday spoken language.

And further, the adoption of Tuscan as the universal was far from a foregone conclusion: by the 16th century it would seem that a sort of unwritten "Courtly Language" had developed as the courts of the Italian states grew closer through marriage, diplomacy, and trade. We unfortunately have close to no idea as to what this language might have sounded like; the keystone work by its leading sponsor Vincenzo Calmeta has been lost to time, leading some to call it a "Ghost Language." Research in the '90s (Drusi in 1995 and Giovanardi in 1998) postulates that the "Courtly Language" was an unstructured vernacular modeled on the language of the Papal Court in Rome, which accepted a high number of tuscan-isms, but was not derivative of it. I however, have always interpreted it as having a Lombard base, seeing as its two greatest sponsors, Calamera and Castiglione, were from Lombardy. In any case, all of that the people speaking this language would have all been tutored in the same same classic tuscan literature, meaning that the overall tuscan influence must have been undeniable. Castiglione, who gives no description of the language, in his writing argues that language is a purely functional tool; and as such favored a completely unstructured form that could even vary from court to court, as long it was intelligible.

The Tuscans themselves were somewhat bemused and occasionally reactionary with regards to the enthusiasm with which other Italians imitated their vernacular. The famous florentine statesman Niccolò Machiavelli wrote a treatise titled "Defense of the Florentine Language," where he explains to a venetian straw-man, named Messer Maffio, how illogical it is for a venetian to write about tuscan grammar, as it would be for a tuscan to write about venetian grammar. Indeed, some tuscan intellectuals developed aversion to any grammatical standardization whatsoever, the argument being that the vulgate was an untamable and should remain living language (the writer Pierfrancesco Giambullari most notably lay this idea out in 1551). Others, instead, like the clique of tuscan intellectuals who came together to form the influential literary academy called l'Accademia della Crusca, published dictionaries to the effect of popularizing a very conservative take on the tuscan dialect.

If the Venetians were most often the targets of tuscan intellectuals' criticism, it is because Venice developed a florid publishing industry that quickly appropriated tuscan standards. Because the people reading and writing books were all extensively tutored in tuscan poetry, it would seem that tuscan just a very convenient a choice for a standard; after all, book publishers were (and still are) interested in making sure that their books are as widely sold as possible, and although the tuscans got defensive about, it writing in tuscan was the best way to make a book widely understood. But the debate amongst Italian intellectuals wasn't over: by the 18th century, the language of literature had already fallen behind the times; the conservative members of the Accademia della Crusca had, through their publication of an influential dictionary, upheld a literary language that was a snapshot of early-renaissance tuscan ideas as interpreted by them. A number of intellectuals like Carlo Denina, Alessandro Verri, and Melchiorre Cesarotti (all northerners) published harsh critiques of the Accademia and proposed the creation of a competing pan-Italian academy. However, hardly had these ideas begun to take hold that Italy was shaken by the french revolutionary armies, who founded the Cisalpine Republic and turned Italy into a french-client state. This stimulated a new school of conservative language purists who sought to return to the roots of the Italian language (perhaps reacting against the French domination of Italy). This popular intellectual clique was rooted in the administration of Napoleonic Italy in Milan (Napoleon's capital), and by 1868 the newfound Kingdom of Italy had placed the Milanese novelist, socialite, and member of parliament Alessandro Manzoni at the head of a government commission to devise a definitive official language to be taught in public school curricula.

Manzoni's commission came up with a standardization which fundamentally consisted of his favorite parts of modern florentine mixed in with upper-class milanese (Manzoni had breifly lived in Florence while completing his magnum opus, a novel of historic fiction called The Betrothed). Manzoni's commission led to the newly constituted compulsory elementary education system adopting a dictionary titled "New Vocabulary of the Italian Language according to the Use of Florence" which although based on the Tuscan literary canon, was subject to the preferences of its authors: Emilio Broglio, the Minister of Education and a native of Milan, and the member of parliament Giovanni Battista Giorgini, who although a native of the Tuscan city of Lucca, also happened to be Alessandro Manzoni's son-in-law.

Italian spoken today, then, is a bizarre creation: originally based on what early modern intellectuals liked most of early renaissance tuscan, it was modernized somewhat arbitrarily by a small clique of intellectuals present in the Italian government in the aftermath of unification (without discussing, of course, the influences of modern media and pop culture).