@ brief history
Probably the most obvious and familiar use of this symbol is as an e-mail separator, and as such it has been in use since it was chosen by Ray Tomlinson in 1972. Its use in business, however, is somewhat more established.
Its first recorded use dates back nearly 500 years, when it was used by Florentine merchants to represent an amphora, a measure based on the capacity of the standard terracotta jars that were used at that time to transport grain and liquid. The sign for it was the letter 'a', embellished in the typical manner of Florentine script.
This symbol was later adopted in Northern Europe with the (still-familiar) meaning 'at the price of'. It was due to this usage that it was included on early typewriter keyboards (from around 1880 onwards). Its continued use was reflected in the fact that it was included in the list of standard ASCII characters in the 1960's. Due to its ubiquitous use in e-mail addresses, its use now extends even into languages such as Arabic and Japanese which do not use the Latin alphabet.
We all know what it means and how to use it, but what is it called? In English, it's most often referred to as 'at', but other languages have far more colorful ways of identifying it.
Swedish and Finnish : apinanhanta : 'monkey's tail'
German : Klammeraffe : 'spider monkey'
Dutch : apestaartje : 'little monkey's tail'
Serbian : majmun : 'monkey'
French : escargot : 'snail' (perhaps this should be in the 'Food' category?)
Italian : chiocciola : 'snail'
Hebrew : shablul : 'snail'
Korean : dalphaengi : 'snail'
Other animals :
Norwegian : grisehale : 'pig's tail'
Danish and Swedish : snabel : 'letter a with an elephant's trunk'
Finnish : kissanhäntä : 'cat's tail'
Hungarian : kukac : 'worm'
Russian : sobaka : 'dog' (many thanks to MrFurious for kindly pointing this out)
Czech : zavinàc : 'rollmop' (a rolled-up pickled herring, which looks similar to sushi roll)
Hebrew : strudel : named after the Viennese rolled apple dessert
Swedish : kanelbulle : 'cinnamon bun' (another rolled-up dessert)
Spanish and Portuguese : arroba : probably from 'amphora' (see above)
English : at or commercial at
French : arobas or a commercial
(Update: 12/20/02. Stavr0 says the 'official' French name is now "arrobe", although everyone already calls it "arobas")
Finnish : miukumauku : 'the miaow sign'
Source : 'Where it's at', http://www.worldwidewords.org/articles/whereat.htm