I've make a bookmarklet out of David Gouch's Javascript implementation of John Gruber's Title Case Perl script.
Drag this link to your bookmarks bar: Title Case
Seems to work ok in Firefox, IE 6 has issues.
**Update 15 Mar 2010:** In Mac OS X 10.6 a better way to do this is via a service that calls Gruber's Perl script. See [Creating a ‘Make Title Case’ Service in Snow Leopard](http://seansperte.com/entry/creating_a_make_title_case_service_in_snow_leopard/) for instructions.