Tormentor667 has set up a monster database for download. Most of the monsters are graphically improved versions of ones from the MRW (they went through the KDiZD process), but there are others as well, usually whatever the community releases. According to his statements:
Submit your own monsters here now and if they are of any worth, be sure that they will be added soon. ;P If you find bugs, problems or things one could improve, feel free to do so and submit the bugfixed wad here as well.
Anyone can send his monster(s) to Torm, or send updates. Unwanted releases can also be removed on request. A worthy resource for any Doom mapper. Here it is.