• The Void in the Underworld; the lowest region of the world
• A place of punishment for sinners; the opposite of Elysium
• The father of the monster Typhon who lies under Mount Aetna
• The online home of a disparate group of free software programmers and their projects

... but mainly it's just a cool name that I liked. (Mythology references from The Encyclopedia Mythica.)

You might like to find out about the people who make up Tartarus, or the projects we've got running at the moment. More or less everything we're working on is available on our version control server, and we run some mailing lists to support the more interesting projects.