AudioCheck.net

Excellent source of audio test and training material. Definitely useful to help learn and train your ears!

EQ Cheat Sheet

Sounds typically heard in various frequency ranges plus instruments and their typical highlight spots.

EQ Your Ears

Another website that lets you focus on training your ears to identify 1/3 octave frequency centers.

Multitrack Stems for Mixing Practice

Nifty site with some public domain multi-track recordings that you can use as a source for practicing your mixing technique and learning how to use a sound system.

Save them to a hard disk and use them in your multi-track DAW as a feed into your mixer.