Other than their remains were found in a cave in southern Siberia and contributed to the genes of modern-day Melanesians, there isn't much known about the mysterious Denisovans. We don't even have much in the way of remains--the above tooth, plus a broken finger bone; and that's it. However, the DNA inside the remains was still in very good shape and could be examined by scientists. While the remains were originally thought to come from Neanderthals, analysis showed that the DNA was far too divergent to belong to that species of humans; in fact, the Denisovans show nearly as much divergence in time as exists between modern humans and Neanderthals! That is to say, both of these groups were descended from a population that left Africa around one million years ago and split into two groups. The ancestors of the Denisovans went east, and the ancestors of the Neanderthals went west, and while they may have met in the middle overall there was little interaction between the two groups. The Denisovans were, then, a third species, one that was neither modern human nor Neanderthal, but something unique.
Could Denisovans once have ranged all over Asia, in the fashion that Neanderthals did over Europe and the Middle East? Given the existence of admixture with Melanesians, it's certainly possible. What did they look like? Given that we've only got a finger bone and a tooth, its impossible to say, although presumably they didn't range any further outside the human form than the Neanderthals did. Maybe Google Image Search knows what they looked like--I got this picture when I searched for "Denisova":
Great googly-moogly! No wonder modern humans bred with them!
3. The Hobbits:
Homo floresiensis, aka "Hobbits" are the most exotic of the fossil human species, and incidentally the most recent--some of their remains are no more than 13,000 years old, meaning they lived contemporaneously with modern humans for 11,000 years longer than did the Neanderthals (at minimum).
Physically H.floresiensis is the most divergent of any known human species. For starters, they're tiny: about one meter tall, an example of an evolutionary phenomenon known as "island dwarfing." Basically, when animals are isolated on island environments (and in the case of the Hobbits this was the island of Flores in eastern Indonesia) they tend to get smaller than their ancestors back on the mother continent.
Of the three human species that shared the planet with Homo sapiens, H. floresiensis is the most likely to be considered a "species" in the traditional view of the term. The Hobbits were, for instance, very likely to be reproductively isolated from moderns, both due to their size and the peculiar features of their brain development. Hobbits had brains one-quarter the size of modern brains, and yet they appear to have been as intelligent as any other early human, capable of making stone tools and engaging in cooperative hunting. Apparently their tiny-but-smart nature was accomplished by shrinking certain areas of the brain while leaving or enlarging others. Given the very intricate nature of brain development in modern human fetuses, it's hard to imagine human-hobbit interbreeding having any sort of happy ending, but who knows? It's not as if we have any evidence for or against. As for the Hobbits' DNA, we know nothing about it--DNA could not be extracted from their remains.
Although the most recent Hobbit remains are 13,000 years old, we don't know when the species became extinct. According to modern Flores natives, there were still little hairy folk around when the Portuguese arrived on the island (16th century). A few optimistic folk speculate that H.floresiensis isn't extinct even now, but lurking deep in the Indonesian jungles in the form of the Orang Pendek (Indonesian for "Short Man"), a hairy little humanlike creature from the folklore of Java (think of the Orang Pendek as Bigfoot's "Mini-Me," if you will).
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