Protector is a defender clone. You fly a single ship against a suspiciously familiar Evil Empire. Bugs, lawyers, bundles, red tape and even more evil creatures will try to stop you.

Protector is free, which is pretty good value for money. It works on DOS, Windows 9x, and Linux.
Click here to download the game(DOS/windows version, 484K), or here to download the source code(113K)

Here's a screenshot.

Two people can play at once!