Having setup and run a DOS/Windows based Accounting system ('Take5 Enterprise') since the mid 80s, I've kept an eye open for OpenSource solutions over the years. As OpenSource moves up the software stack, several solutions have evolved.
There is a Wikipedia ERP page worth reading.
The tools I came across were Postbooks, TinyERP (now called OpenERP), SQLledger, ofBiz, Adempiere, OpenBravo, ...
In April'08 finally decided to get to know OpenERP closer since is covers accounting, inventory management, CRM, customer management and even ticketing. I was wary of the fact that it was really only open source when SVN was opened in Feb'08, and the community is currently dominated by OpenERP partners.
In July'08 I started using it in production for my own accounts (Boran Consulting Sarl). In Jan'2010 I managed to close off 2009 with the v4.2 installation, and started afresh with with version 5 (since migration was too painful).
These pages summarise experience with OpenERP.