Extensions.Frieger.com is described as 'This site contains a searchable database of thousands of file extensions, with information about the most common Windows applications used to open them' and is a Similar Search Engine in the Online Services category. There are more than 25 alternatives to Extensions.Frieger.com for a variety of platforms, including Online / Web-based, Mac, Windows, Linux and iPhone. The best alternative is AlternativeTo, which is free. Other great apps like Extensions.Frieger.com are Product Hunt (Free), SimilarSites (Free), FileHippo (Free) and alternative.me (Free).
When I first switched to Linux from Windows, one problem I had was finding good programs to accomplish some of the same tasks I was already doing. I knew there were plenty out there, but I didnt know how to find them. Linux App Finder was an idea that developed out of my search.
When I am exploring alternatives to a product or service, I often to go Google, type "[product] vs ", and just see what it autocompletes. That usually tells me the most popular competitors. If I want to be thorough I'll do the same for the competitors, etc.