For those that live in Europe and want a powerful search engine that includes low-cost flights (a deal-breaker for me), I recommend Azuon.

http://azuon.com/en/feature-tour

It's incredibly addictive.

Combined with airbnb you can plan some much cheaper and adventurous routes.

The payment plans are as follows:

- €7.89 for 3 months (subscription)

- €12.03 for 6 months

- €16.54 for 12 months

- €19.87 for 1-year

I've subscribed for 3 months now. It's worth every penny.