www.starfall.com
Best of all, it's free! There is a learning curve to use the mouse to click--I made an "account" on my computer for my daughter and made some adjustments to accommodate her: larger cursor, tail on the cursor (to assist in tracking it/finding it), slower cursor. This might help for those just learning the computer. You do need to spend some time teaching the use of the mouse and what to click on (and which button to use to click) but most kids should get it with a little practice.
A lot of people seem to recommend the Jumpstart Toddler/Jumpstart Preschooler software, but I haven't tried either yet.