Echidna
So I'm watching a show on the National Geographic channel about the platypus, because that's how I spend my Sunday nights, when the narrator says that platypuses (There is no universally agreed plural of "platypus" in the English language.) lay eggs. Now, this is a well known fact for anyone who took an interest in odd animals as a child. However, at the same time it was mentioned that one other mammal lays eggs. The curiosity demon teamed up with the deja vu demon simultaneously creating the feeling that I once knew what the other mammal was and creating an insatiable desire to reclaim that knowledge. Thus, I googled.
That mammal is the... echidna. Link included to spare you the 10 seconds required to find the wikipedia page for yourself.
Not only does this fascinating little anteater lay eggs, the rest of its reproductive process is equally, if not more interesting. You can read about it for yourself.
Isn't nature just great?