Alexis Madrigal profiles the man behind the hashtag:

The practice of putting a pound symbol before a key word or phrase or acronym began on Twitter, when one guy, Chris Messina, suggested that Twitter allow them. On August 25, 2007, Messina wrote up his proposal and posted it to his blog.  … The history of any invention is complicated, as Messina’s foundational post details, but this is one case in which some individual human being — in the right place at the right time with the right contacts — came up with something new and watched the whole (online) world adopt it. That’s pretty amazing when you think about it.