- Compiere - http://www.compiere.com
- Openbravo - http://www.openbravo.com
- OpenERP - http://www.openerp.com
- openTaps - http://www.opentaps.com
- xTuple - http://www.xtuple.com
They each have their strengths and weaknesses. Each have a free version, some have commercial versions with additional features. Compiere, Openbravo, OpenERP, and openTaps all have web-based interfaces. All can run in the cloud (usually Amazon EC2) or will be with new releases soon. Hosted versions exist as well.
I plan on publishing more details about each as I continue to test and evaluate against other ERP solutions (mainly Dynamics GP), so stay tuned!