I plan to start an open source billing system written in PHP. It would the Modular Gaming framework as a base (I manage the project).

I was planning on supporting paypal and virtualmin first.

I do have a project site setup at http://modularbilling.com if anyone does wish to contribute - I am happy to give anyone svn write who has any knowledge of PHP.

There is not much on the site now - but I will be commiting the basic code later today hopefully.