[tikhonj]

Hmm, on mine (GNU Emacs 23.3.1 on Fedora) it's M-x htmlfontify-buffer. It came with the Emacs distribution as standard. I suspect htmlize-buffer is either from some external package or from a different version of Emacs.

[freestyler]

It's from an external package called htmlize http://www.emacswiki.org/emacs/Htmlize