YS Flight Simulator is a decent, freeware flight simulator that's only about ten megs.

Hey, I am not going to tell you this is the best flight simulator ever, but it has been around a long time, and you find me a FREE flight simulator under ten megs. Great for nostalgia and a fun simulator. As you can see in the screenshots, there is almost no scenery at all, and to many people, that's a big deal.

You can fly more than 40 types of airplanes. Fly a dogfight, ground attack, formation leader, take off & land the aircraft carrier, try crosswind landing, network vs. play, etc.