I'm working on Premium Poker Tools right now. Basically, it allows poker players to run simulations and find out how often they'll win in different situations. Tools like this exist, but I think there is room to improve on design, features, and friendliness to beginners.
Previously I failed at a startup called College Inside View. There are a lot of sites like College Prowler that have student reviews of colleges, but the questions they address are really broad, eg. "What are the academics like at [school]?". I felt that answering a few broad questions doesn't really give prospective students enough information, and that prospective students have specific questions that they want answered, like "Are the neuroscience professors approachable?". College Inside View tried to be a place that provides prospective students with answers to lots of detailed questions about what a school is like.
I worked on College Inside View for about two years in total. Part of why it took so long is because I was learning to code as I was building it. Part of why it failed is because I wasn't able to get enough content (people answering questions). But I was able to scrape together a solid amount of content for the Ivy League schools in a pilot program. However, I wasn't able to get users, nor was I able to generate interest from high schools or guidance counselors.