I go to Iceland once a year, I fly Icelandair 757 MSP-Rejavik. (nice airline by the way) We go there because my parents love it there, so we end up going. We also go around Keflavik, and I found some great spots of the airport. I've noticed that there is Icelandair there, Myflug, they have a few small props like Dash-8. There is Air Iceland which has 747s, Greenlandair which has the Dash-7, Icebird Airlines, Canada 3000 used to make stops then continue on to Europe with 757s, Icebird Airways, Bluebird, those are the only ones I have witnessed myself, I'm sure there are plenty more.
BTW, if you fly USA-EUROPE or EUROPE-USA and you fly over Iceland, don't you love the vast brown landscape, and you can find a small little farm every 50 miles! That's the best part of the flight, the rugged region of Iceland!