Simply because it's my area, my answer is going to skew heavily towards Early Medieval England, so I ask any fellow numismatists of other periods/areas to jump in.

The answer to your question depends very much on your location, period and currency of choice. In the 770s, for example, we know that Arabic golden dinars were in circulation in Northern and Central Italy, where they were openly used in high-value trade. Examples must have made it to England, likely via pilgrim traffic, as Offa of Mercia mints a replica dinar series in the late 770s. The coin was called a mancus, an anglicisation of the Arabic manqūsh, slang for struck coinage. After the first, imitative issue, however, the coinage takes on a distinctly English element, being minted to the same weight and gold standard, but with English obverse and reverse designs replacing the original Arabic designs. The Mancus wasn't an everyday currency like the silver penny, but was instead apparently a ceremonial coinage minted for specific occasions or displays of benevolence. It did, however, enter regular usage as a term of value, as a sum equal to 30 silver pennies, or one eighth of a pound.

The English silver penny was remarkable for its consistency of silver content and regular value throughout the Early Medieval period, and this was in part achieved through rigorous policing of incoming coinage. Archaeological and numismatic evidence from port wic sites suggests that incoming Frankish coinage in particular was stopped, weighed and exchanged for an equal worth in English pennies. Unlike centrally-controlled English minting, moneying in Francia was 'farmed', which could lead to drastically different silver levels and quality in ostensibly identical coinage. Frankish coins with a sufficient silver value might have been re-stamped with an English die, while substandard coinage was likely melted down for its silver.

Its consistently high silver content meant that English coinage was well-regarded as a functional coinage, especially among the trade partners of the English. Numismatic evidence suggests that the English penny was in heavy usage in Emporia and trade sites across Scandinavia and in settlements along the East coast of Ireland.