It's pretty lucky that LewieP exists, you know. He's rounded up the entire Steam mega-sale in all its glory over here. Yep, that's a pretty serious sale. The most pressing bargain appears to be Super Meat Boy, which is £3 for just the next five and a bit hours. Hop to it!
I'd probably pick up Armed & Dangerous if you've never played it, and I wouldn't get Perimeter 2, because that was a bit rubbish. Also you can't go wrong with Hitman: Blood Money for super cheap. That is a genuinely excellent game.