Is there a free math app?

I'm curious, have you come across any free math apps that can help with various mathematical concepts and problems? It would be really beneficial if such an app exists, especially for students who want to improve their math skills without breaking the bank. I'm eager to know if you have any recommendations or if you've had any personal experience with such apps. Are there any that stand out in terms of user-friendliness, comprehensive content, and overall effectiveness? I'm looking forward to hearing your thoughts on this.