My main three apps I use are:
1. Google Keep - I just use this to keep track of my random thoughts throughout the day or to make my grocery lists, etc.
2. Wolfram Alpha - I mainly use this to help me with math problems. I like this over the web application because it gives step by step instruction without the subscription cost.
3. Bargain Bytes - I use this one to help track sales on steam games that I'm looking to buy. They have a nice feature that lets you see which games are the most discounted.