Here are a few free websites that my kids really enjoy:
Play House Disney - this website offers a wealth of activities based on Disney Channel shows. Everyone from the venerable Mickey Mouse, to the Wiggles and the Little Einsteins is there. There is also a for-pay section of the website called Preschool Time. More about that last website some other time.
Nick Jr. - another great website for kids is Nick Jr. The site does have a few ads sprinkled here and there, but if you don't feel too strongly about the issue, it's a wonderful site for your little ones. My son spends hours playing the Diego games, which are pretty impressive and highly educational.PBS Sprout - Bob the Builder, Barney, Dragon Tales and many more are all featured here. The site also allows your kids to upload art projects and birthday cards to share them with the world. Yet again, PBS rocks.
Thomas & Friends - every boy I have ever met, including my own 3 are hooked on Thomas. A zero cost way of feeding that addiction is to visit Thomas online. It's certainly cheaper than paying $10 for a single, Chinese made, lead painted Thomas toy.
Enjoy these free websites, and if you have any others that you can recomemnd, please let me know.