The Stanley Parable is an interactive story built on the Source game engine. To say much more would be to spoil it...
70% reduction brings it down from £9.99 to £2.99.
Steam daily deal - ends at 18:00 today (31/12/14).
Not the longest of games, but kept me very entertained - for 6.8 hours according to my steam stats.
Highly reviewed, and I thoroughly recommend it. If you're not sure, give the demo a look.