There was recent disussion in a newspaper about this (probably the Times). The accepted names are '"at" sign', or 'commercial "at"', but the suggestion that had the columnist's approval was - on the analogy of "ampersand" - "atpersand". I disagree, but I'm less influential than the columnist.
The reason for my objection is that 'ampersand' (&) is a contraction of '"and" per se "and"', so that the analogy for "at" should be "atpersat".
Note: this is a metalinguistic rant on my part; I'd advise people to use 'the "at" sign' - although I fear that "atpersand" may catch on.
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