The rsync development tree is publicly available by various methods so that you can try out the latest changes and give feedback to the developers.
Note: Since the "head" source is a work in progress it may not compile, or may fail in various ways, though it's usually pretty good.
Anyone may use anonymous CVS to follow development: here are the anoncvs details you need.
You can also browse the development history through cvsweb.
Nightly snapshots of the rsync source tree are also available for people who want to try the latest development version. If you want to track development, then CVS uses less bandwidth and can automatically merge your local changes with our version. However, if you just want a one-off download, or cannot use CVS because of firewall, network or software issues, then the snapshots might be helpful.