These guys have compiled quite a selection of quality, complete games. Actually, to such an extent, one could say that they are one of the most prolific open source game devs out there, even if not all of their works are completely "open".
Most of their games are compatible with the GPX-Wiz, and hopefully the Pandora once Bennu, the language they use for developing their games, gets an official port.
Play as one of 4 fishy characters, all with their own individual abilities and personalities in an atmospheric underwater world. Explore the multitude of scenic aquatic environs and immerse yourself in the lore presented through branching NPC dialogue and interaction, the choices you make being apparent as the game progresses. This of course means that there is a lot of text to read, so I wouldn't quite recommend the game if you just want a quick blast of arcadey action...
Falling under the category of Alt-Arty-Jump-Shoot game, Nether Dimensional Runner has an interesting visual style that is something only truly appreciated when seen in motion. A selection of guns is at your disposal, along with a powerful dash ability that allows you to bounce up walls and through enemies. Gameplay mainly consists of differing levels of either crystal collection or boss battles, but they are cleverly composed as to pose a challenge to the player. The game definitely deserves the recognition it got from the Pandora Angst 2010 Competition
An interesting and seldom seen genre nowadays, Apocolypso is shooting gallery/platforming cross genre game with RPG elements. You play as the iconic lone wolf who has been tasked with defending cities from rampaging robots; gameplay consists of shooting enemies coming at you from the z and x acis, and defending yourself with a shield from bombs and missiles dropped from planes. You also have to ensure that the city you are defending doesn't take overmuch damage either, or else it's game over as well. After completing a level along with its boss, you get to choose from a number of upgrades to help you on the next level. Not an especially long game, only covering 5 levels, but it is of pretty high quality and fun to boot, and being foss, you can always hack on a couple more if the mood takes you :)
And last, but not least, Railroad Rampage, the mandatory Tower Defence. Some might sigh at the genre, but Railroad Rampage definitely contains enough of the Neotron secret sauce (not a very fitting metaphor, in retrospect, if the source was secret, it wouldn't be on this blog!) to make it palatable if not rather tasty. Play as an engineer tasked with getting a train from point A to point B via a bunch of enemies. Defend said train from the relentless attacks of aforementioned enemies, using a bunch of humorous implements. Simple, and fun
And that's all for now, lets hope Neotron stay inspired, keep up their good, and pump out many more cool games