Hashtags are a clever convention invented by Twitter users.  They can be extremely useful, but excessive use of these can make a post harder to scan/read.  If you overuse hashtags consistently, your followers will skip your posts and, eventually, stop following you altogether.  Learn to strike a balance.

Aha Mobile is an iPhone app that gives you real-time traffic information you can use safely while driving.  It includes social media/crowdsourcing elements and it even helps you find the nearest public bathroom!

This is not about the "Plane Down on the Hudson" or the "China Earthquake" kind of "moment," but, nevertheless, this is an example of how Twitter helped us solve a problem we had run into in Mexico almost as quickly as it came up.