There are a lot of ways you could interpret this - up to and including modern forms of spiritualism and the various non-christian religions to have come to increasing prominence, I suppose, though that's really not my area of expertise.
However, outside the Roman empire in the West, the impact of Northmen paganism (the various brands of Viking worship, in essence) was considerable; a great part of their terror was the idea that they had practically no qualms about attacking and sacking christian institutions, something which (at least to an extent, despite the actions of some leaders like Lothair I) was largely respected by Christian rulers.
In terms of a pagan religion which persisted within the West (and excluding the aforementioned presence of un-Christian religions like Islam, which of course spread as far west as Spain), a strong contender would be the cult of Manichee in the later Roman empire, which certainly persisted until the fifth century, over the course of which it faded away - it's particularly memorable because perhaps the most important theologian in the history of the church, St. Augustine of Hippo, was himself a Manichaean before his conversion to Christianity. While Manichaeism's Dualistic model of good and evil returned in various forms (such as Catharism), it largely (though not entirely) faded away in the west by the 6th century; in China and the East, it would persist until the 14th century.
Depends on "significant." The British encounter with Hindus did have an impact, and there were a significant number of people in Britain reading, critiquing, and taking inspiration from, e.g., the Bhagavad-Gita, but very few people actually converted. Interest in Hindu views also spread beyond Britain to other parts of the West (e.g., Hindu views influenced Schopenhauer quite a bit).
It also depends on whether you consider Hindus "pagan." (I think that both modern neo-Pagans and modern Hindus have disagreed on this.) I think there's some evidence of links between Hinduism, Baltic Paganism, etc, as branches of some "proto-Indo-European religion," but whether that makes them both "pagan" is disputable.
Would the Native Americans' original religion, whatever it was called, count as pagan?