Try the demo (no signup required, data stored locally) at https://demo.numerikos.com
I designed this for my 4th-grade son and so far he really likes it.
The main features:
1. Practice problems: generated by rules I've designed particularly for each problem type. They try to avoid trivial problems and create an interesting distribution of problems. 2. An editable dashboard: you can track different metrics or setup different collections practice problems (timed or not timed). 3. Review page: click the star while you are practicing to review a particular problem later. 4. Currently available in English and Chinese (the translation still needs improvement).
I wanted something that allows users to personalize the areas of math that they want to practice more. I will eventually make an algorithm that helps users to decide what to study next, but it will have an escape hatch where the user can override some of its assumptions.
What's next: 1. Support 5th grade math. 2. Release the production version. 3. Support Spanish language version.
My main source of inspiration is Chris McMullen's collection of math workbooks (followed by Khan Academy and Math Academy). My long term goal is to support math at all grade school levels and some college level mathematics and physics. I'd be curious to hear your thoughts on the UI and any constructive criticism you have.