LaTeX on Blogger
mnnttl.blogspot.com
mnnttl.blogspot.com
I wonder though, as plugins (like http://wordpress.org/extend/plugins/mathjax-latex/) exists for users to user MathJax, not alone the same javascript used in the post could be injected to wordpress.com, even without installing any plugins.
Edit: As jamesgeck0 points out, Wordpress.com does support LaTeX: http://en.support.wordpress.com/latex/