I had a pretty good experience with Reviewboard https://www.reviewboard.org
- free, MIT license
- entirely web-based (used it locally hosted)
- primarily oriented towards reviewing code diffs with general and inlined comments and replies support
- it can be used to review non-diffs items as well (as regular file attachments)
- logging in is required, the submitter and all reviewers are clearly identified - might be an issue for you if by "strictly unnamed" you mean "anonymity", but it also has a pretty extensive user and group access control
- it supports integration with revision control systems (used it with GIT and FishEye) and issue management systems (used it with Jira)
- doesn't do any code testing/verification
The UI wasn't extremely intuitive, but with the co-workers' help and the occasional digging into the documentation the learning curve was quite acceptable. IMHO an initial demo/presentation from an experienced user can get new users up and running very fast.
According to the tool's admin the installation and setup were not trivial, especially the integration with the other 3rd party tools.