First, I do not play games that cost money. All of these games below are free and most of them open-source. There are the gaming websites; lagged, poki and other things like that; but I think games that want you to to download them because you want the game, not because you clicked on an interesting picture, are the best games.
There’s bzflag; it’s free, open-source, and has been around since 1993. But it has received almost innumerable updates since then, so it does not look as old as it could. Then there’s the best Minecraft alternative, unless you think MineClonia is better; voxelibre which is a minigame for luanti which you can download as the game engine from the official site. Endless Sky; which is fun but quickly gets tiring if you think dark backgrounds make you uneasy.
Mostly I stick to VoxeLibre and BZFlag; they’re practically all of the gaming tools I need. But if you really need more, I like appel by griffpatch and a few other small games. If you want to try some of the mods I’ve made you can find them here. They are for VoxeLibre.
If you’re interested in my bzflag maps, you can find them here. When updated this page will have screenshots.