WordPress.org

This plugin can generate the PNG images either by using WordPress.com's LaTeX server (recommended) or by using the version of LaTeX installed on your webserver (LaTeX is not installed on most webservers; this method is recommended for advanced users only).

Using WordPress.com's LaTeX Sever (recommended)

1. Install and activate this plugin.
2. If you want to allow LaTeX images in your blog's comments in addition to your blog's posts, go to Settings -> WP LaTeX, check the Comments checkbox, and save the settings.
3. That's it :)

Using Your Server's Installation of LaTeX (advanced)

If you choose this advanced method, you will need several external programs to be installed and working on your webserver, so installation is bit complicated. Many hosts will not be able to support this method.

Server Requirements:

1. Your server must be running some flavor of Linux, UNIX, or BSD.
2. You must have a working installation of LaTeX running. I recommend the texlive-latex-base package together with the tetex-math-extra package. Both are available to most Linux distributions.
3. Either dvipng (provided by the dvipng package) or both dvips and convert (provided by the dvips and imagemagick or graphicsmagick packages, respectively) must installed as well. dvipng is preferred.

Setup:

1. Create a subdirectory called latex/ in your wp-content/ directory and make it writable by your webserver (chmod 777 will do the trick, but talk to your host to see what they recommend).
2. Install and activate this plugin.
3. Go to Settings -> WP LaTeX to configure the plugin and test the PNG generation.