The venture test
Five questions that decide whether an idea is a business or a feature: is there a real customer, is the problem repeatable, is the business model clear, is there a long-term advantage, and is it worth becoming an independent company?
The last one is the useful one. Several things that pass the first four are better off as part of something that already exists — and noticing that early is cheaper than noticing it after the domain is registered.
No download yet — this one is written up here rather than handed over as a file. Ask if you want it.