The following are some great online resources.
Highly recommended. Has lessons, a dictionary of music terms, an online metronome, printable sheet music and more.
Probably the best online tab collection. Tabs are user submitted, so you can't always be sure of the quality, but just look around and read the ratings and comments. Has tabs for guitar, bass and drums.
A Flash based site that lets you look up the fingering of any chord and will play the sound of that chord strumming.
Garage Band and Acid Express are both very useful for creating music on your computer. GarageBand lets you plug in an instrument and record and edit it.