Along with the "standard" discounts, Steam is hosting daily special deals, marking down popular titles like GTA IV ($7.49), Defense Grid ($2.49), S.T.A.L.K.E.R.: Shadow of Chernobyl ($1.99), Operation Flashpoint: Dragon Rising ($19.99), and Mirror's Edge ($4.99). When these expire in 22 hours or so, another selection of hits will take their place.
The event will end on January 3, so there's plenty of time to take advantage of the savings. If you spot something that's simply too good to pass up, tell us about it in the comments.