These are the ones I recommend purely because my daughter enjoys them.
At first we only played the free games but then I decided to subscribe to it for 1 year because the games are quite beneficial for her to learn maths, reading and spelling.
2. Nick Jr
The games are familiar to her because of the characters. Most of the games are free and they have new ones every few months or so.
This one is not free. But it's not expensive either. She learnt her Arabic numbers from this site. I ended up buying the cd set when our subscription expired.
I have a shortcut for each sites on my desktop for her.