For today only, developer Sidhe's retro-inspired brick-breaker, Shatter, is 75 percent off, bringing it down to $2.49. That's less than the cost of a Monster energy drink and, really, everyone agrees you need to stop drinking those things anyway.
Over the weekend, every game in the Deus Ex series, including Eidos Montreal's prequel, Deus Ex: Human Revolution, is 75 percent off – that's $14.99 for the whole bunch, including some item packs and "The Missing Link" DLC for Human Revolution. You can also buy each game piecemeal.
If you like your protagonists a little less gritty, Telltale's Sam & Max games are also on sale for 66 percent off this weekend – $14.99 for seasons one and two, and The Devil's Playhouse.