It is one of the most used signs in Information Technologies nowadays. We use it while seding an email, programming (PERL, PHP…) or in our twits.
But this single character was used for the first time by Mr. Francesco Lapi – an Italian merchant – in 1536, in a letter sent from Seville to Rome. During those days, the @ sing was used to represent an anphora, a measure of capacity based on jars.

According to Giorgio Stabile, professor of the history of science at La Sapienza University, “The loop around the ‘a’ is typical of that merchant script” and the evolution was more or less like this:

The sign, only arrived to English typewriter keyboards in the early 20th century and “it was selected as a rarely used symbol to separate user names from domain addresses by the American internet engineer Ray Tomlinson“. This happened in 1971.

Prof. Stabile says: “No symbol is born of chance. This one has represented the entire history of navigation on the oceans and has now come to typify travel in cyberspace“.

Isn´t that nice?

References:
At sign (Wikipedia)
Merchant@florence wrote it first 500 years ago (The Guardian)