It's musictheory,net, and it's got lots of great lessons. Everything is done in Flash, so the interface is nice and very useful. The best part (to me at least) are the trainers:
- Guitar trainer, where you the computer displays a mark on a fret and you have to tell it what the note is. This will be awesome for me, because I pretty much suck at that, and it is starting to inhibit my playing. There is also a cool brass trainer that displays a note and asks you to press the arrow keys the way you'd press the valves to make that note on a given instrument. Since brass instruments are an offense against the ears of God, I'm not sure why anyone would go here, except that it's a cool idea.
- Interval ear trainer, which plays two notes and asks you to identify the interval. I used to have one of these on my old PDA, and it's the feature I miss most since i dropped that sucker on a concrete floor a few years back. I kept screwing up on the minor sixth for some reason.
- Chord ear trainer, similar to the interval ear trainer except it plays a chord. This one is hard but useful if you're the type of person who tries to make chord charts from recordings.
All these trainers keep track of your accuracy. I ran about 95% on the guitar trainer (restricted to the first five frets, though), 75% on the intervals, and a paltry 40% on the chords (even with the weirder ones removed from the equation).
There's a ton of other interesting and useful content on that site, and it runs from very basic to rather advanced. Cool stuff.