Discovery of puijila darwini in northern Canada, a seal with legs instead of flippers, supports the theory of evolution as a transitional fossil.
Watch this about transitional fossils and creationism:
Theory is not defined as "something somebody thinks is true," as opponents of evolution and magical thinkers would have you believe. Visit this site for a nice description of what a theory is.
Incidentally, Miriam-Webster's dictionary site gets it wrong. Okay, maybe they get it wrong on purpose because of its popular use. But it helps explain how theories, though solid, thoroughly debated and tested, and/or capable of explaining phenomena, can be thought of as "an ideal or hypothetical set of facts, principles, or circumstances."
Here's a nice site from National Geographic which lists seven transitional fossils.
And now a bigger list, just the vertebrates.
Finally, transitional fossils from all flavors of animal.