EFF has just announced a challenge, asking all comers to consider setting up a relay for the anonymizing Tor network. Tor stands for The Onion Router referring to the layers of encryption added with each routing hop. Relays are critical to increase the capacity of the network overall as they are the nodes doing the encrypting and routing heavy lifting. Traditionally clients have far outstripped relays yielding a less than optimal experience when making use of Tor.
There is far more information at the challenge page, including both instructions and most critically a legal FAQ. If you are going to run a relay, whether or not you will do so as an exit relay, you need to be aware of the legal issues inherent in doing so. The FAQ is a good resource to that end and even links to a list of ISPs that are known tolerant of and prohibiting Tor relays around the world.
Tor Challenge, EFF