Part of the development process at work is code reviews. It is a peer review working as a sanity checker, and provides more than one set of eyes looking at the problem. The process itself is fairly simply governed by an excellent source control system in Subversion (with additional excellent tools like TortoiseSVN).
There is an online tool to help with the annotations and code reviews process. It's called Codestriker and provides a web interface and access to the source control through it. Basically the system provides the ability to annotate source code and allow others to read those notes.
It's good for a formalised system, but is a very comprehensive system. This means that it does take a lot more user interface interaction than a simple email detailing any changes. For a large system with many developers it could be more of an invaluable tool, especially for large quantities of comments. It is also useful if your code is really bad and you need lots of comments ;)
The system also has Bugzilla integration, so stick together this, Subversion, and Bugzilla and you have quite a powerful open-source system.