If you are using windows ("gui" tag ?), what about TortoiseBzr ?
If you want some web-based tool ("browser" tag), you can take a look at the WebInterfaces page, which lists a couple of possibilities.
Loggerhead seems quite good :
Loggerhead is a web viewer for
projects in bazaar. It can be used to
navigate a branch history, annotate
files, view patches, perform searches,
etc. It's originally based on
bazaar-webserve, which is itself based
on hgweb for Mercurial.
And here is a demo page, that allows you to navigate, see diffs, ... use the application, actually, on a real repository : http://bazaar.launchpad.net/~bzr/bzr/trunk/changes
For SVN :
For GIT : there is a full/long-list on the InterfacesFrontendsAndTools wiki page.
- Web-based, you could take a look at gitweb ; here is a demo.
- For both GUI and Web based, the list is quite long -- I'll let you go through it by yourself ^^
For CVS : ergh, is anyone still using CVS ? (yeah, I know, I know...)
- Web-based : maybe WebCVS would do ?
You also have plenty of other tootls ; some can even deal with multiple kind of repositories.
For instance, you have InDefero (which does more than just repository-browsing ; it's been described as a google-code clone, actually).