Not Open Source but still/now free to play (these are my favs, I played them through back and forth):

- Trackmania Nations Forever (e.g. via Steam) - https://www.youtube.com/watch?v=aj2u2OrZ2vI

- Starcraft Brood War - though I have not tested it via the battlenet.exe, played it when it still was payware.

- Gemcraft Labyrinth - played it when flash was a thing, but now it is again, e.g. on Armorgames

And, I got games like Portal 1 & 2, Outer Wilds, Osmos, Manifold Garden, Planet Crafter and many more for under ten, some for under five Euros each when they were on special offer on Steam. I once got Just Cause 3 for three Euros.