These are some of the best resources for SSI that I have been able to find.
If you know of better sites which are not listed here please let me know so we can share them with the other 3 visitors which will visit this page.
Server Side Includes
The XSSI Library
SSI from the NCSA HTTPd
Chapter 5 from O'Reilly's book...
Big Nose Bird.com's excellent guide
W3C/Jigsaw SSI Commands
Much more coming soon...