I started working on a product where I really wanted to track some key metrics of the test suite over time. I knew I had some code that "worked" to track the metrics & I decided to spend a few free days working on a nice web app.
I built out some UX ideas using Bootstrap & Middleman. Both these together let me build out ideas for flows and spot any problems early.
I showed a few friends over beers and feedback was quite positive! It definitely thought that even if the product goes nowhere, I'd have a nice project for my portfolio.