A lot of times I start something to learn a new language or technology - I made a little Android game to learn about that, a compiler to learn LLVM, a discussion forum to learn PHP and MySQL, a text reminder scheduler to learn node.js.

None of those I would expect to generate a lot of interest, and there are "better" solutions out there for all of them. But that's only taking into consideration a potential customer's point of view. Each of those has been the best solution for teaching myself what I wanted to learn.