In addition to the daily 24-hour deals, Steam will be running “Flash Sales” on shorter time cycles. The deals available at the time of this posting include Mafia II ($7.50 instead of $30) and Deus Ex: Human Revolution ($7.50 instead of $30).
Other games will also be discounted through the sale period. Now is a good time, for example, if you’ve been meaning to try classics such as Counter-Strike ($5) or Left 4 Dead 2 ($10), or the recent critically-acclaimed Walking Dead game ($19). Bundle deals are also available from individual publishers such as THQ, whose 15-game collection is now selling for $50 instead of $311.
Valve, which runs the Steam service, says it will offer deals on thousands of games. I haven’t found a complete list, but SlickDeals has a forum post with links to some of the major discounts. You can also browse Steam’s Website, though it seems to be buckling from high demand.