I am going to spain in a few months and I wanted to fly between some cities. What are the cheapest airlines to use to book flights in Spain?

All I can suggest is that you look at www.whichbudget.com and the usual flight booking sites as there's no guarantee that one airline will be cheaper than another all of the time

Also look at www.skyscanner.net

For some routes which are very competitive such as Barcelona-Madrid, both Spanair and Iberia often offer very low fares, especially if you book far enough in advance.

AND, before making any reservations look at the rail options which may end up being faster and are usually always city center to city center.

Vueling flies all over Spain and seems to get pretty good reviews (they compare it to Jet Blue). www.vueling.com

The number of choices at several airports in Spain have been reduced by one. Spanair announced that it is laying off 1100 employees and cutting service on 9 routes, including service to the San Sebastian airport (Hondarribia).

If you're travelling across Europe you can check eassyjet.com the cheapest one in europe

clickair have many flights in spain and the rest of europe

Clickair have merged recently with Vueling, new company will kept the name Vueling.

We flew Air Europa from vigo in Galicia to Madrid. Great flight. Good deal. Found it through whichbudget.com and booked on-line. Slick.