Great idea for a discussion! I've tried to put together a list on conferences (although chances are slim that I will be able to go...) Would love to know which ones are really worthwhile.
Some that I know about, but haven't been to are:
Webcontent.gov (for federal and local government web people) always has interesting opportunities:
Web Builder 2.0 - was in Las Vegas in 2008
The Social Media Summit - was in Chicago in 2008
National Association of Government Webmasters - was in Chicago in 2008.
web 2.0 Expo, hosted by O'Reilly was in San Francisco in 2008
South Carolina IT and Web Accessibility Conference was in Columbia in 2008.
An Event Apart - several cities in 2008