I've read a bit about Google's Mondrian, and I've seen Atlassian's Crucible demonstrated - I'm convinced that the tools have value. Are there open-source code review tools? It would be used on a proprietary codebase, so it can't be a public site or service (eg: Google Code's features include some of those of Crucible). We have released some of our libraries as open source, but by and large my employer's codebase is still closed.
Mondrian has been ported to Google App Engine: Rietveld. It's open source (Apache license), you can deploy it on your app-engine account, or port to vanilla Django and deploy on your own server.
Where the code being reviewed is a delta (change to existing code), a tool like WinMerge is immensely useful. I've used ClearCase cleardiff in the past, and WinMerge provides a pretty good open source equivalent.
Check out Review Board (see here for a better review than I could give).
Review Board
Code Striker
Jupiter (site seems to be down as of this writing. Perhaps it has moved.)
If you happen to be a Team System user (which is not open source) then you can use TeamReview which is an open source addin.
There is Rietveld (demo), which is fork of mentioned Mondrian, written in Django (Python) and using Google App Engine, which was written for Subversion (instead of Perforce, like Mondrian), but it also supports (from what I have heard) Git and Mercurial. Then there is Gerrit (announcement), which is fork of Rietveld, which was written for Git.
http://www.codeplex.com/Malevich
A perfect clone of Mondrian, but you can deploy it yourself even on a small workstation. Supports TFS and Perforce.