I'm watching DVDs of the BBC series The Life of Mammals with David Attenborough (see the IMDB listing for more details), which takes a very long and interesting look at the huge variety of animals in the mammalian class.
The first episode deals with the mammals who are closest to those that evolved 200 million years ago, Monotremes (egg-layers) and Marsupials (pouches). There are only two surviving monotremes: the Platypus and the Echidna. I'd never seen or heard of the latter, and it's pretty damn cool looking, in my humble opinion -- a cross between a hedge hog and an anteater.
The Answers.com Echidna page has info on the animal from a number of different sources. The photo here is courtesy of the Ryan Photographic website, which has some really cool Nature shots.
I'll probably put up info on other animals as I watch the rest of the series.