A group of devoted X-Com: UFO Defense fans recently released a pretty complete remake of that classic game. It's called UFO: Alien Invasion.
The UFO series has seen some recent releases in the last few years, including the fun and enjoyable UFO: Aftershock and UFO: Afterlight by ALTAR Interactive. The original game is definetly a cult favorite, and has legions of fans.
This remake looks pretty true to the original game -- perhaps a little too close to the original game maybe (doesn't seem to bring to much new stuff to the table). But hey, an updated and free remake of UFO: Defense is always a good thing. You can check it out over here if you would like to give it a spin.
If you never played the original game or any games from the series, basically, in a nutshell: you are in control of a anti-alien group that is tasked with saving the planet. You have to manage your army, build your base, and research technology to challenge the alien threat. You got to shoot them aliens out of the sky, and they assualt them on the ground with your squad of troops and robo-tanks. The game is very well-designed, and I haven't played this remake, but the original on which this remake was made was a challenging classic.