There is a range of different operating systems available for various machines produced by SGI. The main SGI Operating System home page can be found here.
IRIX is SGI’s SVR4 based Unix.
The IRIX hompage can be found at http://www.sgi.com/developers/technology/irix/index.html
Windows NT – SGI’s Visual Workstation range were Intel-based workstations running NT 4. Although there was a port of NT to MIPS, none of the MIPS powered SGI machines will run NT.
The SGI NT home page used to be found at http://www.sgi.com/developers/technology/nt/index.html. Check out http://www.webarchive.org to see if there are any archived pages left.
Linux – SGI’s most recent range of servers are powered by Intel processors and run Linux. SGI has been quite active in the Linux community, extending existing parts of Linux, and making available SGI developed enhancements.
The SGI Linux home page can be found at http://www.sgi.com/developers/technology/linux/index.html
Linux on SGI – the Linux on SGI home pages cover an SGI port of Linux to run on the Visual Workstations, as well as a MIPS port to run on the SGI Indy.
The Linux on SGI home page can be found at http://www.linux.sgi.com
NetBSD – there is a port of NetBSD to some of SGI’s MIPS based boxen.
If you’d like to see what SGI has been up to in the Open Source community, SGI has a page up within Developer Central.