ClassicUO is a nice open source client for Ultima Online. Need to have the game assets.

Warfork is my favorite

Wipeout-Rewrite

Cannonball

Gzdoom

yamagi quake

I really liked Speed Dreams when I played it years ago. Never heard anyone else talk about it. Warsow and Red Eclipse were also good.

Amagettron advanced en soldat are fun games (not sure if they're open source)

Iter Vehemens ad Necem

When I played open source games around 15 years ago, these were all the same games mentioned. There doesn't seem to be a lot of progress

I love AAAAXY and Xeonjia.

Beyond All Reason is a RTS made with Spring Engine that's an open source implementation of Total Annhilation.