Snapchat. Thought it was a pretty dumb idea till recently. I'm now an active user. To be fair though, I still think the messaging feature of it is fun only as the occasional gimmick. The value tapers off as soon as you pass a certain age (at least that's what I feel). But when they introduced stories, and "live stories" in particular, their value just skyrocketed. It suddenly became genius. Being able to follow a full day of Red Bull's highlights, experiencing a country or city from multiple viewpoints in the span of a couple of minutes was amazing. Watching UFC from the front, back stage, way at the back of the ring, seeing the reactions of people in a curated-ish form that lasted 5 minutes was incredible. Snapchat suddenly became a whole lot more valuable for me.
One other idea. Yo. I don't think anyone's really using it to its potential. Apart from the "Yo" feature, the idea of it as a notification broker is fantastic. I still carry bets that it won't hit "critical mass/momentum" in its current form. Way too clunky.