It may depend on the project, but here's one positive example. I went to the home page, then clicked on one of the featured projects. I've used Chicken of the VNC before, so there I went: http://sourceforge.net/projects/cotvnc/
Thanks! It definitely seems to depend on the project. If there's no repository configured then you can't browse the source. I made the mistake of assuming everything would have a repository though. Some projects (e.g. 7zip) seem to rely on you going to a separate home page and finding the source there instead.