The "@" symbol. . . used by grocers and accountants throughout the English-speaking world to indicate a rate, or cost per unit, as in "10 gal @ $3.95/gal" [ten gallons at three dollars and ninety-five cents per gallon] has become the de facto delimiter in e-mail addresses, separating the user's name from the domain name.
Although the change from at meaning "for a given amount per" to at meaning "in a specified (electronic) location" comes fairly naturally to English speakers, it does not for native speakers of other languages, for whom neither "at" nor @ meant anything until e-mail came around. ...
Following the September that never ended (if it's possible to follow such a thing) as chat rooms and eventually instant messaging made it of the utmost importance to condense and mangle written language as much as possible, english speaking hipsters everywhere began using the "@" symbol to replace the letter sequence ['a','t'] in text, owing to the fact that the symbol is usually spoken as "at". Thus the word "hat" could be written as "h@" and "category" could be written as "c@egory". The irony, of course is that standard keyboards require two key strokes to insert the "@" symbol (SHIFT-2), the same as would be required to simply write "at". Even more ironic - when using the symbol on a telephone keypad it is usually hidden in a series of menus, whereas the "at" text is only two keypresses.
Two keystrokes more when typing it, but a byte less diskspace when saving it;)