Like the ampersand, the ‘@’ symbol is not strictly a mark of punctuation; rather, it is a logogram or grammalogue, a shorthand for the word ‘at’. Even so, it is as much a staple of modern communication as the semicolon or exclamation mark, punctuating email addresses and announcing Twitter usernames. Unlike the ampersand, though, whose journey to the top took two millennia of steady perseverance, the at symbol’s current fame is quite accidental. It can, in fact, be traced to the single stroke of a key made almost exactly four decades ago.

Source: Shady Characters » The @-symbol, part 1 of 2.

If you haven’t subscribed to Shady Characters, I strongly suggest you do if you’re curious about the history behind some characters we take for granted, like the ampersand (&), the octothorpe (#) or the pilcrow (¶), and perhaps lesser known symbols like the interrobang (‽). Keith Houston’s writing style is excellent, and his posts well-researched and annotated. Great contributions in the comments as well.