What kind of software app would you buy?

[Sarada got pantsed]

I am a budding software developer, and I am trying to think of some app ideas that people would like and be willing to buy. What are some web/mobile applications that you think would be useful or fun or worth buying? How much would you be willing to pay for such an app? Do you think it would be better on web, mobile, or both?

Please share your thoughts.