A disclaimer: I live in the U.K., so I can only see what Steam is offering those of us on this side of the pond, and have no way of telling if the various discounts are also available in the States.
For those who also live in the U.K., Steam has some real bargains. Today's deals, available for the next 33 hours, include Scribblenauts Unlimited for £15.40 (33% off), Borderlands games for at least 50% off their original price, Hitman games for at least 50% off their original price, Wargame: European Escalation for £7.49 (75% off), Mirror's Edge for £3.24 (75% off), and Prince of Persia games all for 75% off.
I'm particularly interested in Mirror's Edge at that price, but there's something in there for everyone, and that is true throughout the sale. At the top of the page, you can filter by top sellers, indies, free-to-play, and controller-enabled, so you can narrow down exactly what you want and find a nice cheap game to match.
Another particularly awesome feature of this Steam sale is the Community's Choice, where we're given a list of titles and asked to vote on which we'd like to be put on sale next. The current Community's Choice is The Secret of Monkey Island: Special Edition, which has been dropped from £6.99 to a measly £1.74. But if you'd like to see the next discount applied to Miasmata, Forge, or Waking Mars (my choice – no pressure), head over and vote.