http://tfc.pcgame.com : I.am/TFC. Its got everything, demos, maps, ect... Great site.

http://www.teamfortressclassic.com/: The official Sierra TFC page.

http://plan1etfortress.com/ : Nice site with killer layout and pic of the day.

http://www.fairgame.org/tfc/news.html : TFC news and humor updated frequently.

http://www.tfcentral.co.uk/ : Some lame UK site.

http://geocities.datacellar.net/Area51/Nova/2247/index.html : A new site, but looks promising.