When MIPS Computer Systems was founded, it was a microprocessor manufacturer only. However, to promote their own chips, it was eventually decided to build and sell MIPS-based (of course) servers and workstations. This business did not last long; eventually Silicon Graphics, Inc. (now SGI) bought MIPS to make sure it could depend upon its microprocessor supplier, and terminated the (competing) MIPS computers line.
Since this hardware did not sell in large quantities, it's no surprise that they are quite rare those days, unlike e.g. Sun workstations. Most of the MIPS customers have eventually upgraded their machines to Silicon Graphics machines (with SGI taking care of getting the old machine back to have it destroyed), and very few have hit the hobbyist community.

• RiscOS
  A lot of information about RiscOS and these machines.
• The Lexra story
  Not directly related to MIPS servers and workstations, but rather MIPS lawyers. A story of patents and extortion.
• NetBSD/mipsco
  The NetBSD/mipsco port for the Magnum.
  Currently not working, but I need to test if the recent mips code fixes have made it functional again.

Here is the current MIPS Computer Systems inventory.

• Magnum 3000 (jeroboam)
• RISComputer RC3240