The Timetree of Life is available online now. It allows one to trace the known evolution of species as well as calculate the time of divergence of two species. There is a companion book as well. See also the Encyclopedia of Life.
This is the description from the Timetree web site:
TimeTree is a public knowledge-base for information on the evolutionary timescale of life. A search utility allows exploration of the thousands of divergence times among organisms in the published literature. A tree-based (hierarchical) system is used to identify all published molecular time estimates bearing on the divergence of two chosen taxa, such as species, compute summary statistics, and present the results. Names of two taxa to be compared are entered in the search window and the results are presented on a separate page. Alternatively the last name of an author is entered to find divergence times published by that person. For those interested in published summaries of relationships and divergence times of major groups of organisms (family level and above), see the authoritative synthesis The Timetree of Life.