My wife and I are pretty strict about what sort of media our children can ingest. Our daughter turns into a zombie whenever a computer, phone, or TV is about. Even more concerning is how much she retains from even a single viewing of any media content.
Recently, we vetted the first website to which we were considering letting our 3 year old have regular access. It is a website called Starfall.com. It was recommend to us by some friends who had great success with it. They credit it as a major factor involved in their daughter learning to read early and being well ahead of her peers on reading level now.
The free content includes pretty much everything you would need to get you kid reading. It starts off with the Alphabet, including sounds, word association, and even sign language. It then moves into phonics and basic reading.
I was pretty impressed with it. My daughter loves it, I actually have to make her get off of it. Also, in just the week she has been using it, we have already seen a big improvement in her letter-sound recognition.
We ended up paying the $35 yearly fee to get access to their expanded content, which includes colors, songs, math work, and such. Our daughter liked the Alphabet, but also wanted to be able to do other things, and we figured that if she could start learning math as well, $35 was worth it. I'll admit that I like having a safe, educational website that I can give to her so I can have a moment to make dinner, clean up a little, or just keep my head from exploding.
As I said, I think that the site is great even if you don't buy the expanded content. Be ready to have to set limits on how much your kids can use it per day.