A list of all official patches can be found here.
A huge patch, it adds support for modern Windows, extra renders, multiple bug fixes, and many other additions. It also includes a patch to allow Unreal to run natively on Linux, however it requires Wine to get the files.
It can be found here.
If on Mac, try Unreal Gold OS X instead.
Fusion Map Pack - Adds 6 multiplayer maps to the game and is the only official bonus content for Unreal.
The vast majority of graphical tweaks are accomplished by editing the game's .ini configuration files.
With 'Unreal gold' Steam version edit <path-to-game>/System/Unreal.ini. To obtain 1920x1080 replace the current values for x/y.
Installing the Direct3D 10 renderer allows you to play the game in high-resolution, widescreen, and Direct3D 10. There's also a Direct3D 11 renderer, but currently no significant changes from D3D10 renderer. It is also worth noting that the DX10/11 renderer will not work with the 227 Old Unreal Patches.
Replace the original textures with S3TC Textures and get amazing detail. Do not use these server-side.
Replace the original skins with lore faithful High Resolution Skins
Various modifications, custom content such as characters, maps, plugins, and total-conversion mods can be downloaded and installed for Unreal. Like every game in the Unreal franchise, there's a ton of mods floating around the web on fansites like Oldunreal.
Maps are custom levels that are created using mod tools or editors.
The fastest way possible to run the game on Linux natively is to use one of the Lutris scripts along with Arch Fix one, which fixes the audio issue and for open source GPU drivers it sets the Mesa compatibility to 1999. The script uses OldUnreal's Linux build which can be acquired only by installing the patch through Wine, but in Lutris case it is not necessary as it's included in their repository.
However, as the description notes, you must change the FMODDevices to 27 in [ALAudio.ALAudioSubsystem] section in UnrealLinux.ini (Located in System folder) when you change the settings in-game. If the music does not work, get the 32-bit esound package (Sometimes it can be called esd), run esd in Terminal once, then change FMODOutput to ESD in the same section.