ME1 and ME2 are $5 each - http://store.steampowered.com/sale/MassEffect
@Miltiades - Good timing, what with HL3 seemingly due to be announced officially next year.
By the way, you can get The Witcher 2 on sale from GOG until Jan 2nd - http://www.gog.com/en/gamecard/the_witcher_2
Just set your location in the account options to USA to get the cheapest price. Should be $24.