Musicians and Friends:
The Best You Ever Heard – A music blog by a guy with great taste in blues, roots and bluegrass
Juzzie Smith – Harp player and street-performing one-man-band phenom
Yuri Lane – Beatbox harmonica player with fantastic taste in hats
Pat Missin.com – Covers history as well as technical aspects. Has a nice FAQ and an interesting uncommon questions section.
Overblow.com – Detailed reference on the overblow technique.
Diatonic Harmonica Reference – Large reference on the diatonic harmonica techniques and theory.