Gotten a bit burnt on it lately, but "Frets on Fire" has been so much fun for many many, now 'wasted', hours.
You can pick up the Guitar Hero PS2 controller at a local retailer or online for about $35, a PS2(playstation 2) to USB converter found at Radio Shack for $14, and fire this free software up for a grand total of about $50. (videos inside) Search a bittorrent tracker to find a massive library of songs to play along with. Mindless entertainment, but I think it got us off of our tuffs long enough to get working on things again... it works with a standard qwerty keyboard as well!
a quick search turned up this torrent link via google:
The Guitar Zeros take the thing to a whole new level by utilizing Max with the GH controller to actually make music...
they even include an entire HOW-TO so you can rock it yourself.