Poincaré. Not so much for his mathematical writings (although what I've found in English, or struggled through in French, has been uniformly interesting [if dated, and/or, um, in a language I barely understand]) but for his thoughts on the philosophy and psychology of math. After the already-mentioned John Baez, the first thing I'll implore anyone who bothers to ask to read is "Intuition and Logic in Mathematics," fin-de-siecle thinking and all.

Poincaré. Not so much for his mathematical writings (although what I've found in English, or struggled through in French, has been uniformly interesting [if dated, and/or, um, in a language I barely understand]) but for his thoughts on the philosophy and psychology of math. After the already-mentioned John Baez, the first thing I'll implore anyone who bothers to ask to read is "Intuition and Logic in Mathematics," fin-de-siecle thinking and all.