Here's a website that offers the priceline style flights (where you name your own price) for flights originating in Spain, Paris, Brussels, and London. My friend just got a great deal on a flight Mad-Rome.
http://www.viajarbajoprecio.com/

Happy travels!

Great site, Nevado! It's a shame this Madrid-based site/company doesn't (yet?) have an English version, only Spanish. You can choose your price to locations throughout Spain, but also to/from most major USA cities.

If anyone here tries this system please let us know how you (dis)liked it. Thanks!

Saludos, MadridMan