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.
Next, click on 'Develop.' On the new page under Repositories > CVS, there is a link to 'browse code': http://cotvnc.cvs.sourceforge.net/viewvc/cotvnc/
Agreed, it is definitely not GitHub.