- SuperTuxKart
I really like this game because it's basically just FOSS Mario Kart. It has a lot of gamemodes, an (admittedly simple) kart stat system, many tracks and a lot of mods.
- OpenTTD
In OpenTTD, you transport cargo and people to different places. You can transport them using trains, buses, aeroplanes (airplanes if your American), boats and trams if you add a NewGRF (a mod). It is very addictive and it is quite easy to get into (except the signals).
- Pingus
It's FOSS Lemmings with penguins.
- Extreme Tux Racer
You play as Tux (or someone else) and slide yourself through terrain while you collect fish. It is fun, however I've found that it can get a bit repetitive. You can design levels, however it's a bit hard to do.
- SuperTux
It's 2D Mario with a penguin. BEST GAME EVER.
Xonotic is like mind butter once you learn the flow of it.
Also, friendly correction Minetest is now called Luanti (with Voxelibre mod)!
Luanti(Formerly Minetest) = Game Engine
Voxelibre(Formerly Mineclone2) = Game
^this is the optimum form of the information I intended to convey
Also, afaik, "Minetest game" is still the name of one of the games (and it used to be shipped by default with the engine, though no idea if that has changed and they finally don't include any games in the initial download).
I think the game you can now find in Luanti called "minetest" is a preservation of the mod as it was before the name change to "voxelibre" though I would imagine the actual story is more complicated than that.
AFAIK, Voxelibre was renamed from "MineClone2" which was a fork from "MineClone" which is a different game than "Minetest game"