OK I'm more confused then ever with openbravo now. Their price guide mentions that the community edition is free (but with limited feature set), but when you try and get it on their website it asks you to register for a free 30 day trial of the community edition
:?
There are other options out there, and I think most of them provide some level of free functionality - you'd only start paying once you need to process squillions of transactions, and at that point it would most likely work out much better to pay for it and better organise the business.
ERP seems to work best (only work?) if you embrace it from the ground up, though. Ill conceived implementation have a tendency to break companies:
http://higheredbcs.wiley.com/legacy/college/turban/0471229679/add_text/ch08/fowmeyer.pdf