PPWIZARD is a free and very powerful web development tool (visit homepage) by Dennis Bareis (a Microsoft MVP).

A must have if hand coding, this HTML preprocessor supports #define, #include, #import and a lot of other useful commands.
PPWIZARD actually supports any file types (not just HTML, for example Java, CSS or JavaScript) and can make imbedding Java Script etc in web pages much easier.

It can include external files and could be considered to be a "super SSI" as it can do heaps more than server side includes.

Note that a lot of people recommend and use Javascript for inclusion of common components (for example Navigation bars), not only is this much harder to do and maintain but up to 20% of users have Javascript turned off! I certainly do not wish to turn away 20% of my customers...

It can import from SQL databases as well as from comma delimited and other formats (into a HTML table by default) and allows you to conditionally generate html (for example to create both a framed or unframed site).

You can program it to perform some quite complex tasks such as automatically determine dates or sizes of downloads (or create thumbnails). You can create macros that accept both manditory and optional named parameters as well as positional parameters.

PPWIZARD is multiplatform and supports all Windows operating systems (95/98/ME/NT/2000/XP), OS/2, Unix/Linux, MacOS X, BeOS, FreeBSD, DOS and probably more.

It is programmable and very configurable. Its substitution syntax is very similar to standard html.