On the back of my recent discussions on SQL Developer, at the start of the week Mike Hichwa announced on his blog that SQL Developer Data Modeler is now free!
This announcement was also confirmed by Sue Harper on OTN.
How there is no excuse not to model your applications :-)
More information on this product is available here.
note - us Aussies and probably a whole bunch of other countries spell it "Modeller" - good thing Google Search doesn't care.