So Santa left a brand new computer under the tree this year, eh? Lucky you! If you're spending the holiday setting up a new Mac or PC, it's time to get filled with free software cheer. While the Lifehacker editors take today off to throw back some eggnog, you can point your clicker at a holiday feast of power tips and downloads. Trick out your new rig with our favorite system tweaks and downloads for Windows XP, Vista, Mac, and Linux.