You should look into the websites and mailing lists of various amateur robotics groups, particularly the Seattle Robotics Society.

The simplest way to activate hardware via your computer is by parallel port, which is capable of sinking enough current to drive a transistor. The nice thing about a parallel port is you get 8 digital outs, with no decoding required. This means you can hook up directly to analog electronics. As other posters pointed out, you probably want that to be an optoisolator, connected to a transistor, which in turn is connected to your relay.