This is probably one of the best demonstrations I've ever seen of the true diversity of organisms - in the picture above, representatives of various groups of organisms have been represented at sizes relative to the number of described species in that group. In case you're wondering where the mammals are, we're represented by the reindeer cowering underneath the mushroom.
I received this image from a work colleague - there are a couple of versions out there online that are obviously derived from the same source, but I don't know which is the original version. Contenders can be found at Cornell University and here.
Needless to say, this picture could change as our knowledge of the biosphere improves. The mammal and the bird are obviously too big, while the nematode and the fungus are two that I would expect to increase with time.