Go Fly -- a UK low-frills carrier that was just swallowed by Easyjet (another similar carrier) offered seasonal service from (I believe) STN a year or so ago. You may want to check them out in a few months (they don't list this now).
A list of airlines that fly into Keflavik can be found here:
http://www.keflavikairport.com/airlines.html
Rejkavik is a WONDERFUL town. We took Icelandair. Their service was great!