MimeTeX, licensed under the gpl, lets you easily embed LaTeX math in your html pages. It parses a LaTeX math expression and immediately emits the corresponding gif image, rather than the usual TeX dvi. And mimeTeX is an entirely separate little program that doesn't use TeX or its fonts in any way. It's just one cgi that you put in your site's cgi-bin/ directory, with no other dependencies. So mimeTeX is very easy to install. And it's equally easy to use. Just place an html <img> tag in your document wherever you want to see the corresponding LaTeX expression.
http://www.forkosh.com/mimetex.html
See also ConnectedText and LaTex.
However, MimeTeX has many things missing, so many TeX codes will fail to work with it.