I'd like to buy a scanner to listen to Air Traffic Control (e.g. Tower, Approach, etc.). It would be used mostly at either Washington National [DCA], Dulles [IAD], or Baltimore [BWI]. Would I need a digital or analog scanner to hear these transmissions? Thanks!
The voice communications for air traffic control is all analog (AM to be specific). A scanner that works in that frequency range on AM should be fine. Although a switch to digital has been considered, there is just too much coordination and investment required.
A good reference to find the right frequencies is AirNav. It will list all the frequencies for an airport.
Another good reference for many frequencies, not just airports, is RadioReference. The database there will tell you exactly how each channel is transmitted, so you know what type of radio you will need to listen to it.
Also keep in mind that ATC transmissions are line-of-sight, so you will have to be close to a ground station to hear it. The "rubber duck" antenna included with a lot of scanners is fine but a better antenna will help to improve reception. Also, military aircraft use a higher frequency range, and a lot of cheaper scanners will not receive this.