There are daily sales, quick/flash sales and prolonged discounts. Every 12 hours you can vote on game to go on sale.
You can also sign up for Steam alerts via the Steam Mobile App for games in your Wishlist that suddenly become discounted.
There's tonnes going on at the moment. Borderlands 2 is down to £15 and inventive lovely indie game Natural Selection is over halved to £7.59. ArmA 2 and ArmA 2: Operation Arrowhead - the two games you need to play DayZ Mod - are bundled up and discounted for £15. Remember, the standalone DayZ game is a bit delayed and won't be out for a few weeks in all likelihood.
Some of the lowest prices today are Price of Persia (2008) for £2.49, Deus Ex: Human Revolution for £3.74 and The Secret of Monkey Island: Special Edition for only £1.74. (Bundle the latter with LeChuck's Revent for £2.61!)
There's a huge Indie Super Mega Pack stuffed with games like Bastion, Terraria, Hotline Miami, To The Moon etc. that should cost £140 but has been bundled up for £50.
Valve's bundled up all its games including Dota 2, Counter-Strike: Global Offensive, Team Fortress 2 Holiday Pack and Portal 2, etc. for £25! It should cost over £100.