I swore this one was on RealPlayer's site,but it's definitely not there now if it was. You get tetris-like pieces of wall that you rotate and place on an alien landscape. As ground is enclosed, you generate people inside, like a little colony. You get guns at certain times, and stuff to defend the colony with. Aliens come from all around and try to destroy your walls - if they breach a wall, your little people get sucked through the hole. The idea is to enclose enough space to generate x-number of people to win the level. I found this one terribly addictive.
...And I just remembered where the other game was that I was tryign to think of. So one down, and one alien-blasting-tetris-meets-qix game to go. I'm sure I'll think of something else I'm missing too. I always do.