Quora is usually a good place for finding stuff like this. Usually someone has already asked a question for startups similar to X (assuming X is popular enough)

Quora is usually a good place for finding stuff like this. Usually someone has already asked a question for startups similar to X (assuming X is popular enough), and one of the answers is usually rather comprehensive. For example: http://www.quora.com/What-startups-are-working-on-Uber-for-X http://www.quora.com/What-startups-are-working-on-Uber-for-X?