Best Open Source Top-Down Space Shooter : Super Space Invader

Lasers, rockets, shields, health regeneration, pixels. You know. The good stuff. Just nicely packaged with levels of just the right duration and hardness, persistent upgrades, shaders, reverb...

Two buttons are enough to play, since that was one of the themes of Ludum Dare #34, in which the game came #74 among 1638 jam entries.

Super Space Invader Review

Supser Space Invader was made with LÖVE (v0.9) and thus runs on Linux, OS X, Windows.

The source is available on GitHub under zlib/libpng.

Galaxy Forces: Moon Lander Action!

Galaxy Forces is an open-source moon lander shooter single-/multiplayer game hosted on SourceForge, it is nearly finished.

It is unique of its kind in offering global hiscores and achievements. Replays and AI enemies also available.

Try it. It plays on Win, Mac and Linux.

Strife: Veteran Edition

Just in case you can stand having Steam DRM installed in your system, I recommend taking a look at Strife: Veteran Edition. Strife is a Shooter/RPG based on the Doom Engine and dating back to 1996. The developers for this new remake had the wits and sensibility to use the Chocolate Doom engine as the main code base, which is, of course 100% licensed under the GPL. With so many top quality Free Software engine remakes around here, it really astounds me why so few developers choose to use them on modern HD remakes and ports (I'm looking at YOU, upcoming remake of Heroes of Might and Magic III not using VCMI).