AlternativeTo might be an interesting tool when doing competitive research. The new site describes itself as a a way to find better Windows, Mac, Linux and online applications by submitting the app you want to replace. It's a community site that relies on user submissions, so the number of alternatives is limited in some areas. Having a look at the Basecamp alternatives, for instance, only shows MS Project and the RedMine web app when I looked. But more popular app suggestions are abundant, e.g. alternatives to MS Word.

This could be a good resource with time for collecting examples to reference when doing a competitive study for a web app. One to watch.