I've been evaluating a number of code review tools (mostly free ones), but they all seem to be aimed at reviewing patches before they are committed. This wouldn't really fit within our workflow with Subversion, so I've been looking for alternatives that better support reviewing committed revisions instead of just diffs. Any recommendations? I would prefer free or inexpensive tools.
Have you looked at Atlassian Crucible? It is built for both pre-commit and post-commit reviews. I've used it for post-commit and the functionality is pretty nice.
Pricing balloons quickly - it's nearly free ($10) for self-deployment and 5 users or less, but then gets quite expensive if you need more than that. You didn't mention your team size so I apologize for suggesting expensive software if you have more than 5 team members.
ReviewBoard is pretty much like you said, its a review system that's not really hooked into your SCM, but could be scripted in a post-commit hook. (ie when you commit, the transaction is fetched, diffs run and these then sent to ReviewBoard).
An alternative is Redmine. As you view the repository in redmine, you can create tickets with links to the revision history (ie usually by viewing the diffs in redmine, and creating the tickets from offending code directly). You need the Code Review plugin for this to work though.
PS. both are free, open source.