Valve has started its Steam Summer Sale today, which runs from June 19 to June 30. There are some great games on sale, with 14 hours or so left on games like Far Cry 3 which is down from $29.95 to $7.48 (or 75% off) and The Witcher 2 with 80% off, down from $19.99 to just $3.99. These deals change every day, so keep checking back for the latest sales.
There are also flash sales which change every 8 hours, where for the next 8 or so hours, Half-Life 2 is just $2.49 down from $9.99 - but you'd be crazy to not already own that, right? Anno 2070 is enjoying 75% off, dropping from its original price of $34.95 to just $8.73.