See here how to install it on your system.
A Debian package (version 2.1) is available thanks to Mohammed Adnene Trojette and is extremely popular! It appeared in the sarge (oldstable) distribution, so it is most probably available in your package management software (like apt-get, aptitude, synaptic...).
Here is a binary for Intel processors (version 2.0), and here is a binary for PPC processors (version 2.1). Thanks to David Keech for providing us with the Mac PPC binary!
You can download the Texturize 2.1 sources or the older Texturize 2.0 sources.
Thanks to Dennis Lichtenthäler, there is an ebuild for Gentoo Linux and its package management system, Portage. Here is a howto about Installing 3rd party ebuilds.