AntTweakBar looks and behaves worse than 3dgui (the internal GUI system used in Sauerbraten and Red Eclipse), but at least it's a standard library, so people who know how to use it don't have to learn how to use yet another GUI system.

Tesseract uses its own revamped UI system, which is more powerful and flexible than 3dgui. Most notably, it relies on images less often, which makes theming easier.