I recently attended the QCon 2008 (http://qconsf.com/conference) international software developer conference in San Francisco and was very pleased with the quality of the speakers - and the quality of discussions that were participated in by the other attendees.
One aspect of the conference that was a very pleasant surprise: the percentage of attendees from Europe, South America, and Asia. A very diverse collection of top-shelf talent.
The No-Fluff-Just-Stuff symposiums are excellent 3-day conferences that focus on transfering knoweldge that is immediately useful:
http://www.nofluffjuststuff.com/home.jsp
The two Seattle Code Camps I've attended (and presented at) were similiar in that they are put on by developers, for developers.
I'm curious what other conferences others have found most useful for their professional development?