It started just a minute ago, and will be updating throughout the week with various mega-wonga-ultro-offers. Also, 30 dudes/dudettes/dude-miscs will be awarded the top 5 games on their wishlists every day, so you should totally load that thing up with the most expensive games you can find.
There're even bigger discounts, if you're prepared to collaborate with your fellow human beans.
In terms of the standard sale today, Steam's offering Prototype (quite good), Shank (not so good), RUSE (ace), Deus Ex collection (50% incredible), Alpha Protocol (divisive), Counter-Strike Source (you know), Borderlands (mostly great), Sam & Max Season 3 (much better than the earlier seasons, I'm told) and the Indie Energy Pack (ludicrously good value for 5 interesting games) for Reduced Monies.
There's also the giftpack sale, wherein you can get multiple copies of different games (Borderlands, Ruse, Deus Ex or Half-Life 2 today - again, all change tomorrow) for even more crazy-cheap. E.g. £40 for 6 copies of Borderlands, or £90 for 6 copies of Ruse. So, I would strongly suggest you lot get together in comments below or on the forum and form into bargain-tribes who can then play with/against each other in said games' splendid multiplayer modes.
Oh, and that mention of getting something for your 'nemesis' on the Steam front page - reckon that's just a gag, or another Valvian mystery-generator?
You should also totally buy Beyond Good & Evil for £2 from GetGames, or you're an idiot.