I was reading Beatrix Potter (I love children's books) and came across the name Thomasina, a feminine form of Thomas. When I looked it up, I found that it had the nickname Tamsin.
I sort of fell in love with both these names, and if I were to use it, I'd give a daughter the full name and just call her by the nickname. What do you ladies think? I haven't heard this name much at all, but it's got a lot of "old lady name" charm.