Database answers , it's a company which has chosen to offer, online, over 200 database schemas (mostly proven and applied in production) on the net.
to go directly to the schemas, here's the link: Free Database Models Page .
I personnally have used two schemas for stock management and invoicing, and it worked great. I was able to provide solutions to my client, write from the database schemas, modified and customized to the application needs of course.
enjoy, and avoid reinventing the wheel!