Like a lot of fans I bought MDK on Steam only to find it was a bit of a lackluster release, so here is my fan patch that adds a bunch of stuff missing from the Steam version and also makes it more compatible.
3DFX The patch updates the game to use the 3DFX version in conjunction with the nGlide wrapper. Use nglide_config.exe in the game folder to set your desired resolution (it supports pretty much anything, I recommend setting it to your desktop resolution) and other miscellaneous settings. This should fix any problems related to the original D3D version of the game such as visual glitches. If you are still having problems with graphical glitches, try updating your graphics card drivers.
Steam Overlay Support You can now use the Steam overlay while playing the game to discuss what MDK stands for or other topics of interest.
Menu Slideshow Originally the menu had a slideshow which showed credits (who worked on the game) as well as screenshots from the game. This was missing from the Steam version but this patch re-adds it.
Ending Cinematics A lot of fans like me were disappointed when no Billy Ze Kick music video popped up at the end. This patch adds it again for the proper MDK experience. It also adds a missing cinematic showing Kurt and Bones escaping.
INSTALLATION INSTRUCTIONS (for setup) Simply follow the on-screen instructions to install the patch. Make sure it's being installed in your MDK Steam directory. The Setup installs into the default location so if your folder is NOT in the default location you need to manually browse for it.
INSTALLATION INSTRUCTIONS (for manual installation) Open the zip archive using your archive program of choice and extract the contents to your MDK Steam folder (by default it's "C:\Program Files (x86)\Steam\steamapps\common\MDK"), answer yes to replacing MDKD3D.EXE and then you're good to go. To uninstall the patch you would have to delete the game folder and reinstall it from scratch.
If you have any questions regarding this patch, leave them below and I'll try and answer them asap.