I have played many games like spacechem before.  It seems like every year or so, I run into a flash puzzle game that consumes my mind for a time.  The flash version of spacechem was something about alchemy that Dillon recommended, and only consumed me for about a day... because I beat it.  But spacechem appears not to have that problem.

It's a programming game, which I love, but it also has a lot of polish: leaderboards, factories and such.  And a surprising number of my friends have played it.  Maybe I just have really nerdy friends, but maybe PC Gamers are attracted to programming games.  Who knew!