@ is perhaps the iconic symbol of digital culture and computer use. More so even than the ubiquitous “www.”.
The @ symbol we all use in e-mail came about very early on in the development of computer networks and inter-personal collaboration. In fact it was used on some of the very first Digital Equipment Company (DEC) Programmed Data Processor (PDP) machines (DEC PDP-10s) that were connected together to form the ARPANet around 1969. ARPANet was the predecessor of today’s Internet. Ray Tomlinson working at BBN (who built some of the packet switching systems that form the basis for the Internet) extended a local single computer programme which left messages for other users so you could pass messages between nodes on ARPANet by addressing a user with user@host.
The Edinburgh AI Department ran the DEC PDP-10 for the UK AI community which was connected to the ARPANet. As a PhD student in 1972-1975 I used the local PDP-10 and one at Stanford for my research work in AI planning, and I recall using the send message program and user@host frequently to leave messages for other AI planning PhD students I collaborated with such as Gerry Sussman at MIT and Earl Sacerdoti at Stanford University/SRI.
See for example http://inventors.about.com/od/estartinventions/a/email.htm.
The @ (“at”) symbol though does have quite a history and goes back far earlier than the internet and the emergence of digital culture, see http://email.about.com/cs/emailhistory/a/at_history.htm.