Trackers:

• ModPlug Tracker - A Windows native, robust and very well-rounded tracking program that supports most old tracker formats (MOD, STM, S3M, XM, IT). If you only want to play song files, try ModPlug Player. If you want things even easier, you can use the ModPlug Plugin to play the songs straight through the browser.
• The Official Impulse Tracker Homepage - the classic tracker for DOS and Windows 95/98/ME.
• The FastTracker II resource page - the other classic tracker for DOS and Win9x/ME. (the official page no longer seems to exist)
• BuzzMachines.com - Buzz Tracker is an exremely powerful and ground-breaking modular tracker program. From this page you can download the tracker and oodles of plugins.
• Psycledelics Community - Psycle is a new tracker that's looking very interesting. It seems like the best of both worlds of IT and Buzz combined.

Tracker music archives and sites:

• The MOD Archive - a nice collection of music files.
• Nectarine - A demoscene-dedicated archive with daily broadcasts

Tracking tips and resources:

• The Zen of Tracking - a good starter package of information for people who want to learn more about tracking.
• The Audiofile Tracking Mag - an excellently put together tracking magazine with oodles of advanced tracking tips and articles. Expired, but worth checking out regardless.
• United Trackers - An organization to keep the trackers informed and the scene alive. Check it out.

Samples and sound files:

• Soundwave's Sample Collection - A massive archive of samples; also contains links to other sample archives.