According to British Web analysis firm CacheLogic, BitTorrent currently accounts for an astounding 35% of all Internet traffic, that is, more than all other P2P networks combined. Now, that's what I would call an impressive achievement considering that a year ago the technology was merely hitting mainstream.
Although P2P file sharing has the reputation of being widely used by pirates for distributing illegal software, BitTorrent in particular is being used nowadays to decentralize the distribution of legal downloads that go from Linux ISOs, to large game demos, popular video trailers, eBooks, etc.