I just found the TeX plugin for Serendipity (s9y.org). It lets you draw cool math symbols like
The Serendipity plugin is on this page, or you can download the zip file directly.
You'll also need the mimetex program, which actually creates the gif image from the text expression.
I downloaded the mimetex binary directly, from here. I moved it to the /usr/src/ directory.
There was one trick with getting it to work with serendipity. That was making sure the "images" folder inside the plugin's folder was writable by the web server. Either chown apache images or chmod 777 images will work. This is necessary because the plugin will cache the image once it's created, so it only has to use the mimetex binary file once to generate that first image, and from that point on it will always use the cached one. It uses an md5 hash to generate a unique string to identify each math expression. Very clever.
