Although we started it all off, I think by far (mainly Eastern) Europe has the worse problem with hooliganism. A lot of firms over there are heavily into right-wing politics and neo-nazi idealogies. Obviously some of the UK firms are into that as well, but it seems to be more prominent on the continent. Here's some videos I found on Youtube:
YouTube - Football Hooligans
http://www.youtube.com/watch?v=RcBxsnPuegc
YouTube - Armada Rijeka Football hooligans ultras
http://www.youtube.com/watch?v=_pj2yHUSQOc
And keeping it a bit closer to home, here's a news story about a Chelsea v Milwall game from the 90's. Both Chelsea and Milwall have pretty hardcore firms:
YouTube - Chelsea Vs Millwall