X-COM. X-COM. X-COM. X-COM. X-COM. X-COM. X-COM. X-COM. X-COM. X-COM.
And then there's Frozen Synapse. It's the lovechild of X-COM and DEFCON.
EDIT: Also, Fallout Tactics did not live up to my expectations (and they're pretty low to begin with). If you go into it ignoring the fact that it's a Fallout game you might enjoy it.