http://opensourcecms.com/
check out the Portal(CMS) section and look at all the demos. check out the admin areas too. there are tons of themes/templates for all the CMS scripts that look really professional. they are very easy to maintain and update as well.
i use Joomla on my site, which is nearly identical to Mambo. i think PHPnuke is also popular.
quite honestly, your site looks pretty drab compared to what's out there. BUT, if you're site has good content i think that trumps the design (unless it's horrible to navigate, hard to read, etc). if a site has good information that i need or want to read, that comes first, imo.