Hi guys. I've been using Volutive and so far it managed to satisfy my invoicing needs. It also has a network option so you can access the same database from different workstations. For its cost(free), this program is definately worth it.

Here's the link, and click on Volutive1 which is freeware.
http://www.binarema.com/

If you run the following search you will come across several ERP solutions:

http://freshmeat.net/search/?q=erp&section=projects

It would help if you can specify your requirements so users can assist you better.

what about quickbook?