See the 10 Open Source Opportunities entry in CIO Minute.
The OS and Virtualization territory is already staked out. Not sure this will be a real opportunity space.
Enterprise applications (CRM, ERP, BI) are scary. Scary because these are purchased by CIO's. CIO's are herd animals and -- of course -- an open source enterprise package will never be acceptable.
Clearly, there's a lot of room to make money here by shoving SAP, Oracle and TriZetto Group aside. I think that you can easy implement a simpler application than these legacy behemoths.
Technology (Mobile, VOIP, Security) This has a lot of room for growth. It's enabling or foundational technology. Lots of space here.
Tools and Browsers are totally crowded with freeware. I'm not sure there's any money-making opportunity here. ActiveState and WingWare are who you start competing against. Then you've got to work against Oracle, IBM, the Eclipse Foundation, Sun's NetBeans. Mozilla, Opera, etc.