ABCmouse.com is a great instructional website for toddlers, pre-k students, and even kindergarteners or first graders. It starts with letter and number recognition, shapes, basic counting, etc. and builds up to reading with sight words. There are fun games, a reward system, an interactive classroom, and a store for the kids to buy prizes with their tickets. It's very attractive and friendly. It's FREE for teachers!
ABCmouse also has free apps for the ipad or iphone! Check out this article about the free apps, including links for each of them, via Smart Apps for Kids. You HAVE to sign up for this site's daily emails. They will let you know when some apps are free for only a limited amount of time, and they'll keep you informed on great educational apps.