All Things Open is one of my favorites. But I'm biased - it's held near where I live, and I've attended and/or presented there numerous times. But beyond that, I like that the focus is on, well, "open" things - open source, open hardware, open web, open culture, and related topics.
LibrePlanet
StrangeLoop: interesting talks, by engineers and people working on the problems themselves
!!con: short talks on small-ish, interesting projects