Vueling have put many domestic Spanish flights on sale from just €17 one-way during the summer. For example, fly from Ibiza, Spain to Barcelona for only €17 one-way.
Vueling are running an amazing sale where you can fly from Vienna, Austria to many European cities for only 1 cent one-way over the summer. Also works in reverse.
American Airlines have put Atlanta, Detroit, Minneapolis or Cincinnati to many Spanish cities on sale. For example, fly from Atlanta to Ibiza, Spain for only $396 roundtrip.