I believe the two most popular are:Launch.
Demo has been around for, I believe, two decades and many great products saw the day of light at Demo.
Furthermore, Twitter launched at SXSW, possibly because it's engaging crowd got them early traction. They have many great sessions for startups, and are supposedly hugely popular to visit for startups.
Also, there are The Y-Combinator Demo days, but they're closed sessions for the selected companies and possible investors and mentors.
I'm sure there are lots more that are local and/or niche.
Being at a company that has presented at both Demo and TechCrunch50, I'm biased towards the TechCrunch conference. They're both great, but it seems like there is more buzz around TC. And since there is no fee for presenting there seem to be more of a startuppy feel to it. A sense of community, so to speak. Demo also has a lot larger companies that will launch products, and is not solely for startups.