UT2004 has a Linux port, I believe developed by Icculus[0] but it's pretty long in the tooth. Requires libc5, an old version of SDL, and OSS for sound.
Regardless, it'll still run on modern systems if you acquire old libc binaries[1] (and preload them), use SDL 1.2 (possibly a patched version[2] which enables Alt+Tab) and start the game using aoss[3] for sound.