Skyscanner can also construct somewhat of surprising itineraries by piecing together tickets on multiple separate LCCs that involve non-protected transfers, which is something that other price search engines (Hipmunk/Kayak) or online travel agencies (Expedia/Orbitz) don't really do. Most fare engines won't say "hey, if you're trying to get from London to Ljubljana, how would you feel about taking Easyjet LGW-BRU and Adria BRU-LJU?" — but Skyscanner will.
And they have a fairly cool "I want to go from [x city] to [anywhere]" web UI that will show you, for a given weekend, what are the cheapest countries/cities you could visit? The UI is very good, arguably a bit better than the Google Flights fuzzy date/trip search.
It's a great tool for exploring European fares. But I'm not sure how they make money — there's not a lot of revenue in air bookings.