Here's a gathering that nobody has mentioned: Porcupine Festival in New Hampshire. It's not a "developer conference" per se, but there are very solid tech talks there. It probably has a more dense schedule of events than a typical tech conference, but it has a lot of material that might be considered "off topic" at a tech conference, such as political theory, practical tips for living freely (gardening, handing police encounters, etc), and, of course, a giant bonfire.
There are also really wonderful tech talks, and this has pretty much become the focus of the event. In addition to dev talks - which range in topic from crypto to mesh networking to solar power monitoring - there's also material on 3D printing, drones, beer brewing automation, high-tech gunsmithing, and radio communication.
There's also a beer exchange cum key-signing party which has become a hillariously awesome tradition.
It's great fun and a great place to learn things you didn't know you wanted to learn.
Other than Porcupine Festival, I'll also echo other people's suggestion to attend PyCon. It's more of a cultural event than a dev conference per se, but it's a really great gathering. And being in Portland, it's surely going to be quite a party.