The SSI Mini-Tutorial

Enabling SSI's in Apache requires only a few things:

Compiling in mod_include into the server or using LoadModule to include it as a DSO
having it enabled with the Options directive (i.e. Options Includes)
a method of specifying how the server knows which files to parse
- specify as a MIME type: AddType text/x-server-parsed-html .html
- use the XBitHack directive and set the permissions (i.e. chmod u+x foo.html)

`Server Side Includes: A Mini-Tutorial`

Introduction
Including other files
Echoing Individual Request Variables
Echoing All Request Variables
Setting Request Variables
Date Formatting
Flow Control
Remote Host Detection
Cookie Detection
Browser Detection
Conclusion

Note: there are additional features supported in Apache 2.0, the documentation for the updated version of mod_include is here. Among the cool new things are user definable SSI delimiters, user definable error messages and other good stuff. See also the Apache site's SSI Howto.