Computers

They're really good for some stuff. Don't get carried away though, they can be bad for your personal hygeine.

GNU/Linux

It's an operating system, and it's free. Really free. That's quite a remarkable achievement. What's even better is that in so many ways it outclasses Microsoft's offerings, though the end-user support has a little way to go before it can really go toe-to-toe. Still, a blessing to geeks the world over.

Peer to Peer

You can get hold of almost any film, piece of music, or popular TV show ever made, from the web, for free, within a few hours. Ethics aside, that is almost unbelievable.
Check out edonkey and bittorrent for the cutting edge of the filesharing community.