This one's for both teachers & students (& of course, anyone else interested in evolution and evolutionary trees): the Evolutionary Genealogy website :) It's a site that "seeks to promote the teaching and acceptance of the biological theory of evolution by emphasizing one of its great lessons: that life on Earth is one big extended family, and therefore we are related in an exact way to not only every other living thing, but also to every thing that ever lived."
(And hat-tip to PZ for pointing me in this direction, and also towards the paper describing how the Time Tree was developed & how it works.)
Evolutionary Genealogy has information on the 'great tree of life', and how to use it to look at relationships between any 2 organisms on the tree; the concept of evolutionary genealogy, and a link to the Tree of Life project. (There's also a shop with some rather wonderful t-shirts - I can't decide between the border collie & the kitten...)
Related to this is the Time Tree - an on-line calculator that will work out the degrees of seperation/relatedness for you, plus data from both nuclear & mitochondrial DNA, plus links to the sources of the data used in the calculation. Bearing in mind that the dates are perhaps best described as 'fluid' - see PZ's comments thread for an interesting discussion on this), the Time Tree's looks like it could be a useful teaching tool if one wanted to set an exercise in developing a phylogenetic tree for a group of organisms :) (NB I did this for 'cat' & 'human' & the ensuing table of molecular timing data has these words at its head: Laurasiatheria/Euarchontoglires. Looks strange (where did my cat & human go?) - until you remember that neither cats nor humans have been around all that long, & the most recent common ancestor would have been a placental mammal that in all likelihood looked nothing like either modern species :)