I seem to have a mild form of synesthesia. In my mind's eye I see digits and numbers in colors, and years in a spacial form (like a set of waves, with the begin of the decades being on the hill of a wave, and the end in the valley). Until very recently I thought that everybody would handle numbers this way, then I happened to hear of Synesthesia.
Although it occasionally helps me with remembering stuff, I don't have the powers to make difficult calculations in my head, or recite a phonebook. :) I still remember a phone number I had to use once when visiting London in the late eighties, though.