This Dr. Horrible's Sing-A-Long Blog photo contains
boater, leghorn, panama, panama hat, sailor, skimmer, straw hat, fedora, felt hat, homburg, stetson, trilby, and campaign hat.
There might also be
dress hat, high hat, opera hat, silk hat, stovepipe, top hat, topper, beaver, snap brim hat, beaver, and castor.
The third track of Dr Horrible's Sing-Along Blog's Commentary: The Musical, Ten-Dollar Solo. Main Part by Stacy Shirk... and gatecrashed by Neil Patrick Harris.
This is sort of a fan's response to Dr. Horrible. In it, the fan dressed as "Dr. Dreadful" applies to be Dr. Horrible's sidekick. Made by Mark Andrich.