Under the editorial guidance of Roy Cook a number of new entries in philosophy of mathematics have appeared on the Internet Encyclopedia of Philosophy. As I understand it, the aim of this site is to present relatively short summaries which are accessible to a wider audience, esp. undergraduate students, than some other options.
Check out these recent entries:
Bolzano's Philosophy of Mathematical Knowledge (by Sandra Lapointe)
The Applicability of Mathematics (by me -- more shameless self-promotion!)
Mathematical Platonism (by Julian Cole)
Predicative and Impredicative Definitions (by Oystein Linnebo)
A list of the all of the philosophy of mathematics entries can be monitored here.