In my secondary school time I spent a lot of time playing UFO X-Com game and it’s successor UFO Terror from the Deep. Even now I think it was one of the best games I’ve ever played - even if it is turn based. Developers mixed a lot of great ideas into this game and made it challenging but very addictive. And today accidentally I’ve found PocketPC version of this hit created by Russian company SMK Software (salutes). And it is free.