Just for fun: Time tree app.

Blair Hedges and Sudhir Kumar of Penn State University have developed a Timetree of Life data base that tells when two different species last shared a common ancestor. For example, by plugging in 'rose' and 'frog', and then choosing a particular representative species of each from a drop down list, I found that those two organisms diverged about 1.4 billion years ago.

In case you desperately need to know when butterflies and snakes diverged, but you aren't near your computer, there's now a free iPhone application that does the same thing.

I installed it on my phone, and it works like a charm.