Secret Maryo Chronicles
Secret Maryo Chronicles started as a Super Mario-clone, even using the same graphics as the older 2D Mario-games. After a while the project started replacing the "stolen" graphics for new original assets, which made the game much more interesting and original. Couple this with some nice OpenGL effects and SMC has lots of potential and is very fun to play. Last game update was 2009 though, and I'd love for the graphics replacement to be completed. There is still activity on it's homepage, so here's hoping!
The old FOSS-classic 2D platformer SuperTux got a 3D version at some point. SuperTux 3D is a full 3D platformer, similar to something like Mario 3D World.
An interesting project with nice graphics. I'd like to see more of this; it could turn into something really good with some tweaking.
A massive project with very active development which just suddenly stopped, jClassicRPG had a lot of promise. All sorts of inventive stuff like intelligent population to dynamic ecology would have been really fun to play around with. A lot of work left to do on this one sadly, so hopefully it will get a second life some time.
Ultimate Zombie Fighters
Next, Ultimate Zombie Fighters would be a fantastic addition to LAN-parties as a quick teamwork-based zombie survival game. Development stopped early though, and the game could use both features and graphics.
Coldest is a mech-combat simulator, where teams of giant warmachines take on each other in large-scale battles trying to destroy the opposing teams base. Not only good looking, this could also be a fantastic multiplayer title. There's bots with AI as well as some very advanced graphics done. It's homepage have a message explaining why development is frozen, so this game really isn't abandoned as such. I decided to mention it here anyway though, since it seems so promising. Perhaps with a bit of help development can continue.
There we go! As mentioned, I have more of these coming, so expect another list soon!