One game I had fun with that I haven't seen on the lists here is
Sploit! which is REALLY inexpensive (at $2.95) and has a free trial of the first two levels to boot. I don't think it's the newest of games but i enjoyed getting through it. Actually I was kind of sad when I got to the end because, well, it was over. I wish it had a feature like some games to design your own battlefields. That would enhance the replay value.
But hey, for $2.95 even as is I thought it was worth it!