Ask HN: Must-attend events for a tech startup?

Tech conferences and events are expensive, time-consuming and more often than not a waste of resources if you're a startup without infinite time and money. However, some of them turn out to be worth one's while. I'm just interested in which. To get the ball rolling, I attended Web 2.0 Expo Europe and LeWeb last year (fortunately not paying). Expo was good for the tech side of things and to get a feel for general opinions about e.g. advertising revenue models. LeWeb was mostly a waste of time but for a couple of interesting talks (which were streamed) and people (whom I may well have met elsewhere).