DEFCON, and Microsoft's PDC seem pretty interesting to me. Any other really good ones out there?
Microsoft's TechEd is a great conference for (Microsoft technology) developers.
I think it depends what kind of development you do and what you are interested. Both the IEEE and ACM put on conferences on a number of topics on a regular basis.
I haven't had the pleasure of attending one, but any ALT.Net conference/gathering/clan meeting or whatever you want to call them.
I really enjoy SDWest. The STROUSTRUP & SUTTER ON C++ SUPER SESSION is really great if you're into C++ -- it's educational, fairly deep and certainly entertaining. The conference does a great job of covering skill levels and languages. Get the VIP pass and make a week of it.
I personally have really enjoyed the DevConnections set of conferences, a great chance to cover a variety of topics.
I have been attending local CodeCamps (.NET focussed). You should check to see if there are any in your area. They are free and have great content -- they accept all presenters, so if you know something well and want to present it, you should.
They are a .NET version of O'Reilly's FooCamps (and there are also BarCamps) -- I haven't been, but I bet they are similarly good.
I'm a big fan of MiX for Web 2.0 topics and designers. It's a small conference, so you don't feel like you're hearded with the rest of the cows. Plus it's at the Venetian in Vegas. Need I say more?
Not a "must go", and it's mostly game-related, but if you're anywhere near Dallas in August, QuakeCon usually features a talk by John Carmack. This year he spoke for three hours about everything from game console development trends to rocket science (literally, he's part owner in a rocket company).
The No Fluff, Just Stuff conferences are excellent. As the title implies: no vendor driven content, all stuff you are interested in. I've been to three of them and they have all been very fruitful.
I found DevConnections last year to be pretty disappointing. Probably only 1/3 of the presentations I went to were any more informative than reading a magazine. A lot of the presenters didn't have answers to questions at all.
In fairness, a lot of the questions were around WPF and other 3.0/3.5 features that no one really knew much about yet - but I think that's the responsibility of the presenters - to know more.
I've enjoyed both the Future of Web Apps and RailsConf Europe conferences.