Simple Invoices is a neat little PHP/MySQL project that we have been following that can manage billing and invoices for a small business.   It is very easy to use and is perfect if you have outgrown the excel spreadsheet management but are not large enough to justify use of a larger full featured accounting package.   Another advantage is that this platform can be made to be accessible from anywhere.

All you need to do is:

• Install (or have it installed)
• Set up yourself as a biller
• Set up your clients
• Add your products and prices
• Add any applicable tax rates
• Create (and even email) invoices
• Mark them as paid when the time is right

There are many reports that can be used to track your invoice generated data.  Currently this is a fairly new project but the development is taking place at a pretty rapid pace.  Expect to see many new features in the near future. There is a small communtiy area so the support factor is there and active.
As with any open source project, the code is available and it can be modifed to suit your purposes.
If you are interested in a tool like this we can help you to get this going.
More informaiton can be found at www.simpleinvoices.org