I’m a self-proclaimed synesthete; the topic came up recently here on SDMB after the “Am I the Only One” thread.
Re: Number/letter genders. For whatever reason, odd numbers are male, even ones are female, consonants, male, vowels, female (and y is male, no matter how it’s used.)
As for memory, I’ve got the freaky memory thing going too. Especially things like phone numbers, etc. I had a boyfriend 13 years ago who bought a car, and because he got a kick out of my memory, told me the Vin #. To this day, I remember it. (I just doublechecked it against Carfax.com, and I do indeed remember it correctly.)
One of my oddest memory “talents” relates largely to reading. A perfect example: I am re-reading the Chronicles of Thomas Covenant for the first time in 15 years. As I was rereading yesterday, I was trying to remember the exact sequence of events for something that happened later in the book (because, unfortunately for me, I can’t remember things like that nearly as well). I knew that the information I was looking for was on a left-hand page, three quarters of the way down the page, about 4/5 of the way through the book. I constantly index information this way - it makes it extremely easy for me to remember things I’ve read - in my mind’s eye, I can “see” the page, the location, and with a little thought, can usually re-read the passage. Obviously, sometimes I can’t, but then it’s quick work for me to look it up.
I also remember words very clearly - often, I can tell you when the first time I learned a particular word was. I learned the word “lee” from The Secret of NIMH; “apoplectic” from “The Mirror of Her Dreams”; etc etc. It’s really rather odd. This might also explain my uncanny (some might say annoying) ability to remember song lyrics.
I also mentioned in that thread that I assign locations to music - if I hear a song, I almost always have a place that it makes me think of. It’s always a place where I heard the song before, but usually not the place where I heard the song for the first time. It doesn’t always happen - apparently the association doesn’t always stick, but 99% of the time it does; if I hear a song that I know well, it’s more than likely got a “place” assigned to it. These associations have no rhyme or reason to them - there’s no significant event related to them; it’s not like it’s the first time I heard “our” song with a boyfriend, or anything that major. Just somewhere along the way, I hear the song, the association is made, and I don’t know it until the next time I hear the song.
I don’t know if these quirks are related to synesthesia or not, as I said, I’m only self-diagnosed, but I wouldn’t be surprised if they were.