Answer: The most common word in use is arroba. Interestingly, the word has been around a lot longer than e-mail has.
The word arroba is believed to have come from Arabic and has come to refer to a unit of weight equal to about 12.5 kilograms (about 27.5 pounds) or 11.502 kilograms, depending on the region. Arroba also came to refer to various liquid measures varying from region to region. Although such measurements aren't standard or official, they still get some local usage.
The arroba has long been sometimes written as @, which is a kind of stylized a. So when e-mail developed, it became a natural step to simply use the term arroba when the symbol was used in addresses as well.
The term la a comercial is also sometimes used to refer to the symbol, just as it can be referred to in English as "the commercial a." That term was sometimes used to refer to the symbol on typewriters before it became known more widely for its use in e-mail.
It is not uncommon to use the word arroba when writing e-mail addresses so they are less likely to be copied by spam robots. Thus if I were trying to slightly obfuscate my address, or if I were using some sort of a typewriter or device that couldn't handle the standard symbol, my e-mail address would be spanish.guide arroba about.com.
Incidentally, the period used in e-mail and other Internet addresses is known as punto in Spanish. So, if were verbally telling someone my address, it would be spanish arroba spanishguide punto com.