Pocket PC gamers may have not been able to enjoy the quality commercial software most other platforms have to offer, but they definitely can't complain about the quality (or quantity) of the freeware games they are constantly getting. Turbo Invaders 2.1 is a prime example of said phenomenon. It's a faithful, yet slightly enhanced, version of the original Space Invaders with pleasing visuals and addictive gameplay. Download it here. It's absolutely free.
Related posts: Sierra adventures for PPC, Warring States, SkyForce: the Pocket PC shmup, Top 5 PPC emulators