You might want to look at [opensourcecms.com...] they have demos of various CMSs. Just be aware that these are default installs. I know the CMS I use, postnuke doesn't have an appealing default install so be sure to check out the official sites of each one and look at example sites as well.
Personally I use Postnuke (not to be confused with phpnuke)and I am happy with it. Others that are commonly mentioned Joomla/Mambo and Drupal.
You might also want to mention what you are looking to do so people can give you better recomendations.