Khan Academy Kids is a free app created by the Khan Acadmey people. It focuses on letters, numbers, sequencing, patterns, left/right, critical thinking, emotion recognition, reading, science (learning about nature), music, movement, etc. DD is four and loves it.
I also like Leo’s Pad to develop executive functioning skills. It was created by Stanford early education researchers.
We also bought several preschool workbooks for DD to practice writing her letters and numbers. I need to sit beside her for those though since I need to read the instructions to her.