the Sega Complete Pack on Steam. For $90, you can grab all of Sega's PC games currently available on the platform; a list that includes Empire: Total War (and all its expansions), Space Siege, The Club, Sonic & Sega All-Stars Racing, Aliens vs. Predator and a whole lot more. (Sorry Mac users, but almost all of these games are still only available to the Windows crowd.)
If you're looking for something a bit more cost-efficient, Steam is also offering Aliens vs. Predator alone at half price, for $24.99. But why not chip in the extra $65 and pick up more Total War games than you could ever play in a single lifetime? That seems like a sound economic decision to us.