RT is very full-featured and customizable, although it has a few rough edges and can take some time to learn. It's open source and is also available in as a hosted service. http://bestpractical.com/services/hosting.html
FogBugz is decent -- not as flexible as RT, but may be a bit easier for non-techies to get started with. It's also available in both hosted and installable versions. http://fogcreek.com/fogbugz/