I recently wrote about the ABC game at the Fisher Price site.  That game is perfect for toddlers (and preschoolers), and I found another ABC teaching site that is great as they move into the preschool years at Starfall.com.  It has different areas for each letter with animation and great sounds.  It includes both uppercase and lowercase as well.  It is really well done- check it out and let me know what your kids think of it.