It’s a highly addictive puzzle game with a ridiculous number of levels.
FYI, these are all browser-based. The OP didn’t specifically mention that he was interested in just downloadables, but I thought it was implied…or was it?
I guess my original intent was download games, but browser games are getting pretty good now a days, so I really don’t mind those being included.
Little flash games probably not so much, but the bigger ones that you can save or you have an account or something like that is just fine.
Terribly addictive, tried it last night and ran out of fuel saving colonists in the Lalande system 
I’ll possibly be laughed at for suggesting it, but I like Seiklus - if you ever enjoyed playing Manic Miner, you’ll probably like Seiklus - it’s not exactly the same, but I think it pushes many of the same buttons. It’s very simple (you can’t even be killed), but manages to be charming and interesting.
I have to second, third or fourth (whichever it is) Kingdom of Loathing is awesome and provides some really good laughs. Not to mention I love the little stick figures.
An Adventurer is You!
Good! Now I know I won’t be the only one dragging his butt to bed five hours late because of this game. 
Good grief. This game is so addicting it ought to be illegal. I haven’t done any work in the last 2 days because of this game.
I might add that this game has a lot of Japanese dialogue, but you can download an English language patch at http://agtp.romhack.net/doukutsu.html
Okay, only [del]two[/del] three more hours, and that’s it. I can stop the game any time I want to…
The best game ever made www.freeciv.org
Dwarf Fortress is one of the best games ever made, and its still in Alpha. Check it out, just don’t exepect to learn the ropes your first few tries.
Hmm…interesting.
Oh yeah, the english patch! Thanks for mentioning that, Subway Prophet.
I know you’re probably just getting back to work and all, but Pixel’s earlier game, Ikachan (english and japanese downloads here), is also quite good! Worth a look for sure.
My favorite thing to do at the moment is to sit in a protected spot near a stargate, ambushing every pirate that comes out.
For those who love risk, I recommend Kdice
You know, it borrows a lot more from Space Rogue than from Elite.
Thanks. I’m not getting any work done today, anyway. 
Pah, I power up my tritium upgraded fusion drive and take to the nearest pirate outpost with my omni-directional laser
Of course having died many times I may try your way too 
Well, that game has proved I havn’t magically become coordinated over the last few years. There’s only about 4 buttons to hit and I still can’t get it right. 
Still I was making good progress untilI had to fight the suitcase who became a giant frog.
The problem with that is you eventually run out of outposts. 
I had a great trading run going for a while. The second system had this ice asteroid and the stuff they harvested went for big bucks in Eridani.
Then there’s the time that I found an illegal drug lab and a smuggler port in the same system…