I mentioned the Historables app a month or so ago. It's ready for download on iTunes, and it's free (and has no advertising). It's good for the 3-6 year old set.
Watch the video below about a few of the characters, Marie Ant-toinette, Teddy Bear Roosevelt, Spaniel Boone and Lionardo Da Vinci. Using music, humor and very cute characters, this app is aimed at the 3-6 year old crowd and is fun AND educational (yes, my kids and I tried it out - they were giggling and fighting over the mouse).
Disclosure: I do know the creators, and I'm posting this here because I trust them and have been hearing about its development for awhile now. The app will be for both iPad and Android, but it was launched first for the Apple market. You can try some of the games online at whereverland.com.