[I got started on this post via Digg , a social version of Slashdot. Lots of science stuff pops up over there.]
Discover magazine's March issue has a great account of the discoverers of Mimiviruses, an extraordinarily large virus ( and maybe one of many ) which may have relevance to understanding the origins of life. Most viruses are very stripped-down infection machines, and their DNA or RNA contents are so spare that studying their deep evolutionary history via sequence comparisions is very difficult. The genome of Mimivirus, in contrast, is larger than that of some parasitic bacteria (in fact Mimivirus might be independently alive, and at a minimum blurs the boundaries. See Wikipedia ), and the full genome has been a gold-mine of evolutionary remnants.
Mimi's genome contains elements with similarity to many of the major DNA viruses known as NCLDV (including poxviruses), plus a whole lot more. This observation by itself strongly suggests that NCLDV viruses are descendants of an ancestor at least as complex as Mimi-- thus, possibly independently alive-- with their excess genetic material lost during reductive evolution in the transition to their current parasitic niche. In addition, several mimi genes and gene control elements seem to be equally related to eukaryotic and archaeal sequences, suggesting that the Mimi-like ancestor, and its NCLDV descendants, deserve their own branch in the tree of life next to eukaryotes, archaea, and bacteria. (This is controversial. See this technical comment and a well-written summary here. In my opinion the evidence for independent ancestry looks pretty good.)
I guess what's coolest to me as an outsider is how intimately locked in parasites are with evolutionary history. (Of course, we humans are parasites on plants.) Many evolutionary innovations have come as an attempt either by hosts to evade parasites, or for parasites to evade host defenses. Secondly, oddball organisms often have a lot to tell. Lastly, the nature of mimiviruses as probably barely alive means that the next few years could bring some pretty detailed ideas about what it means to be alive, here on Earth and elsewhere.
UPDATE: Some evidence based on conserved structure for an early origin of viruses