The smart dudes at AbelCineTech have released a really cool "Field of View Comparator" for determining what different lenses look like on different cameras. Say you have a full frame Canon 5D Mark II, and want to see what a 35mm lens looks like compared to the new Panasonic AF100.
magnified sensor (like the Canon 7D).
Check out Philip Bloom's Blog for more details, and video explanation of full frame vs crop sensors.
Also, the ARTEMIS iPhone app is an excellent way to see different focal lengths on the go.