Na Piobairi Uilleann The Dublin Pipers Club—Alma Mater of piping.
Cumann na bPiobairi The Seattle Pipers' Club including the Uilleann Forum
The Session Want the dots? Want to know where a tune has been recorded? The Session is a good forum for exchange.
JC's ABC Tune Finder Another excellent resource for tracking down versions of tunes. The site can return them in ABC, Midi, PDF and other formats.
Uilleann Obsession Pat D'Arcy's excellent site for all things Uilleann.
Chiff and Fipple Mainly for afficionados of the Tin Whistle, but includes the Uilleann Pipe Forum.