We've featured DIY projector screens before, but as far as DIY projects go they can still get a bit pricey. If you're got a nice, flat wall, you can use this method to create a great screen on the cheap.
Projector Screen Paint for the home theater. Less money and better image than a pull-down projector screen! Will use this one if we end up putting the home theater in our living room (good for rooms with more natural light).