The whole world (on Twitter) is watching messages shared with hashtags like #jan25 and #egypt, where people are self-organizing online content about topics of global interest. Click on one of those terms, or anything with a # sign before it, on Twitter and you'll be brought to a search page filled with all the messages using that tag. People use hashtags on Twitter to share fun topics of conversation as well, they are regularly among the most popular topics of conversation on the site.
But what was the first hashtag ever used? According to serial social technology innovator Chris Messina (on Twitter), it was the above: #barcamp. Used in August of 2007, the #barcamp hashtag was intended to bundle conversation about the global technology unconference gatherings called Barcamp that Messina helped found as well. Now hashtags are showing up everywhere. On the sides of busses, (referenced) in The New York Times and this weekend Audi will bring a hashtag to the Superbowl for the first time.
Ironically, Twitter founder Evan Williams once told Messina that hashtags were too nerdy to go mainstream and that Twitter would use machine learning to group Tweets together automatically by topic instead. The use of hashtags before Twitter was most common (though quite different) in IRC, which was part of Messina's inspiration but admittedly a nerdy milieu.
Hashtags are a little thing, but they have become big deal.