Thanks for the list Who's Wee Dug. The only one missing is the original (UK) one:
http://www.dwcon.orgOh and I think the Polish guys are having gatherings too:
http://pratchett.pl/As to what goes on... a whole lot of fun for Discworld fans of all ages. There are usually lectures, discussions, panels, games, plays, music performances, story telling, interviews, parties, dinners, costume parades & contests, and much more. Throw in the social time outside of scheduled events and the hundreds of new friends you'll make, I heartily recommend going to at least one in your lifetime

[I've been to four in Australia - one of which I helped to run - and one in the UK]