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I only ever played QuakeWorld multiplayer, never single-player. For that I've used ezquake and fodquake.

ezQuake has a lot of features, like updated particle effects, custom HUD and IRC and voicecom integration.

Fodquake is more minimalist, but is has a nice high performing multi-threaded implementation. It's great if you want to squeeze out every last frame on a really old machine. It doesn't even support single player of offline mode.