Subversion read access
Subversion is a replacement for good'ol CVS. You can get clients for most platforms at http://subversion.tigris.org/, and there is good documentation like the Version control with Subversion manual, by some of Subversion's authors.
You can get read access from the main URL https://alleg.svn.sourceforge.net/svnroot/alleg/. While you can access this link directly and will see the files, that's really boring - instead you want to use a Subversion client to check out the whole source and make local modifications in order to send patches. Example:
svn co https://alleg.svn.sourceforge.net/svnroot/alleg/allegro/branches/4.2
(for the stable 4.2 branch)
svn co https://alleg.svn.sourceforge.net/svnroot/alleg/allegro/branches/4.4
(for the stable 4.4 branch)
svn co https://alleg.svn.sourceforge.net/svnroot/alleg/allegro/branches/4.9
(for the unstable 4.9 branch)
If you need write access to the Subversion tree, please ask for it on the Allegro development mailing list.
SVN::Web Interface
Theres's an SVN::Web interface to the SVN code at http://svn.tomasu.org/index.cgi/allegro. It uses a mirror so may only show commits up to 5 minutes later but the site may work faster then the ViewVC accesc at Sourceforge.
ViewVC Interface
You can use a web interface to browse the repository directly at http://alleg.svn.sourceforge.net/viewvc/alleg/.
Test results
Daily SVN test results can be seen here: http://allefant.com/tests.py
Changelogs of the Subversion tree
The SVN repository also has a commit hook for CIA, a system for tracking open-source projects in real-time. The CIA web interface provides an RSS feed and notifications in the #commits channel on the Freenode IRC network. You can monitor Allegro activity at http://cia.vc/stats/project/alleg.
