UT2004 Download - Includes all patches and fixes (including the ones below); this install is fully modernized.

UT2004 CD Key Changer
OpenALSoft with voice chat fix - Fixes 64-bit audio pitch and also fixes voice chat on Linux (setup instructions).
UT2kx Regcheck - Adds/fixes UT2004 registry entries (ut2004:// URI, .ut2, .ut4mod, install path, CDKey, etc.).
Large Address Aware patch program - Fixes these issues with UT2004.exe/UnrealEd.exe/UCC.exe: UT2004 QoL patches - Fixes the FPS cap on Windows 10+, among other things:
UnrealEd slow selection fix (Incompatible with UT2004 QoL patches above.)

Press to generate a UT2004 key