Visual Annotate is a nice tool which enables you to annotate a file in ClearCase using an easy to use GUI (instead of using the cleartool annotate command line and trying to figure out what the output means).
It provides a clear, color-coded visualization of where each line comes from (which version and which user). There are many features and customization options, and also integration with IDEs and, with the latest version, the ability to integrate with various Issue trackers.
The most useful feature in my eyes is the ability to see where a change really comes from – if a certain line is the result of the merge, You can see (as a tooltip) the source of the merge, helping you understand who actually did the code change, and where. This is a real time-saver.
In their latest release, they also offer a free edition – you can use the tool for free 20 minutes per day. I suggest you go and check it out.