International Trombone Association
Trombone Catalogue at Hickey's Music
TAP Music Sales - great source for trombone CDs.
Sheet Music Plus - Some great sheet music finds here.
TromboneExcerpts.org - Trombone excerpts in GIF format, with MP3 audio examples.
Trombonis Australis - Gregory van der Struik's excellent website dedicated to Australian trombone music.
Finnish Works for Trombone - Valtteri Malmivirta's annotated list of Finnish music for trombone.
Music for Brass - Catalogue of solo and ensemble sheet music for brass, available for download.