I'm sure many of you already have it, and have noticed, but SpaceChem is currently £3.05 on Steam, until the end of today's deals. If you haven't bought it, buy it.
I didn't get it when it came out due to it being the winter exam season, and then forgot about it until today.
In the last half an hour I've gone from head-scratching confusion to defeatism, to desperation, to disappointment, to yells of delight, quickly followed by anguished bellowing. Five minutes ago I was back at the yells of delight stage when I finally completed a complex production quota.
It's really, really good - well worth the measly three quid (about five dollars?) they're asking, and you don't need to know anything about chemistry to get along.
Here's Quinns extolling Spacechem's virtues, in case you haven't read it.