Here are some trombone links that I find especially useful:
General Trombone Websites
International Trombone Association – Trombonists, you should join this one. Quarterly publications (many backissues available online), discounts on instruments, travel, and more.
Online Trombone Journal – Good articles, discussion forums, trombone classifieds.
The Trombone Forum – Large variety of discussion forums.
UtahTrombonist.com – Resource for trombonists in Utah. Good place to find private teachers and professionals in the area.
Trombone Page of the World – Current News, lots of information, some video.
Brass Players in Film Music – Information about studio musicians. Includes personnel lists for many movies, behind the scenes video, and more.
Musical Chairs – Free list of current jobs (all instruments).
Stacy’s Trombone Site – Free list of current jobs, lots of information on current orchestra sections, almost always a mouthpiece sale.
Trombone Excerpts – Listen to tons recordings of most major excerpts, great resource!
Orcheseek – Large list of the world’s professional orchestras.
Musician Wages Blog – Great articles on surviving in the professional world of music (recording, touring, gigging, etc).
Brass Musician – Online magazine for brass players. Lots of articles.
American Brass Quintet – One of the world’s leading brass quintets. Uses a bass trombone instead of a tuba.
Utah Trombone Authority – Trombone quartet, based out of Salt Lake City, UT.
I Tromboni – Trombone ensemble, based out of Canada.
Douglas Yeo Trombone Web Site – Professor, Arizona State University. Former Bass Trombone with Boston Symphony for 27 years. Many articles, musings, etc. Tons of great stuff!
Will Kimball, trombone – BYU professor, UTA, excellent pedagogue, great research on breathing and trombone history.
Trombone Lessons by Tom Gibson – Many interesting video podcasts concerning different aspects of the trombone.
Joseph Alessi – Principal, New York Philharmonic. Many solo recordings.
Jay Friedman – Principal, Chicago Symphony.
Weston Sprott – MET Opera trombonist. Playing tips and interesting articles.
Wycliffe Gordon – Amazing jazz player who knows the horn inside and out.
David Vining – Professor at Northern Arizona, very fine player, includes narrative of recovery from focal dystonia.
Public Domain Music Sources
Brass Music Downloads – A lot of great arrangements for various brass instrument combinations, including trombone alone. Lots of Renaissance stuff.
Kreusch Sheet Music – Tons of scores from most major composers. Most are piano scores, but there are also orchestral and solo scores.
International Music Score Library Project – IMSLP also has a ton of public domain scores. Browse by composer, time period, nationality, instrumentation, or genre.
The Horn Guys – These guys have a bunch of stuff for sale, including instruments, music stands, metronomes, tuners, etc. Also some good advice on traveling and other information.
Essential-Music-Practice – A lot of good advice and resources for effective practicing.
TromboneTube – A site dedicated to YouTube videos of the trombone and other brass. Very many videos, decent browsing/search feature.
Everyone should own a metronome, but in case you cannot get to one, here are a couple free online applications.