Actually, the "Choosing the Key" Section in the above post is incorrect. A diatonic harmonica works in 5ths. The draw key is always a fifth away from the blow key. Playing harmonica using and bending the draw notes is refered to playing cross harp. When you play and bend the blow notes, you are playing straight harp
Therefore:
A "C" harp will play "G" cross harp
A "D" harp plays "A" cross harp
An "Eb" WILL play "Bb" cross harp
And "F" plays "C".
Straight harp plays in its own key. For instance, to play straight harp in the key of E, you need an E harp.
The two fullest and sweetest sounds keys in my opinion are Bb and G. Bb plays a F cross harp and G plays D cross.
If you do not know how to play cross harp, but you can whistle, you are not far off! Imagine whistling the highest note you can and then the lowest really quickly. Now try that with the harp, but do it as you suck in on the 3rd hole, instead of blowing out. Its that fast change that helps bend the reed to play a lower note.
If you want to hear some of the most amazing harmonica ever, you need to listen to 2 artists: John Popper of Blues Traveler, and Stevie Wonder. I listened to "Hook" by Blues Traveler, and "Boogie on Reggae Woman" by Stevie Wonder, and I knew I had to learn how to play harp. I have been playing for 2 years now, and I've loved every minute of it.