I've done a couple suggestion threads in the past and they have always come up with plenty of interesting answers - however it is all too often that someone provides a huge meaningless list of hundreds of games or that game names are dropped without any explanation. So I thought it would be interesting if I got people to recommend just *one* game per post, and if they had to write a little paragraph about why it is so good. Try and avoid major mainstream games - they already have big marketing budgets to help them out! Things like alpha-funding projects, indie games, little funky things like Kerbal Space Programme would be ideal.

Example:

My recommendation would be OpenTTD, I've come back to this game so many times. It's essentially a game where you build railway networks that will eventually contains hundreds of trains whizzing around and will soon become completely ridiculous with gigantic backlogs and absurdly sprawling track layouts. What I love so much about this game is that there are so many levels of enjoyment to it - just getting your first little track from a coal mine to a power plant going and seeing it all in operation is lovely in itself. Making this little tracks are fun, but then you move on to larger networks that soon spiral out of control, and after that there is an immense amount of depth that can be found by looking online to find incredibly complicated designs and systems. Essentially, the game can be enjoyed at any level.

Download: http://www.openttd.org/en/download-stable
There's a wiki here with tutorials (but it seems to be down just this minute): http://wiki.openttd.org/OpenTTD