I am a member of the Dr. Horrible Live Journal community and people have been doing a ton there.
http://community.livejournal.com/dr_horriblesing/ (If you just want icons go here:
http://community.livejournal.com/dr_horriblesing/tag/icons )
I have a few on my LJ I love... but since NPH is the new love of my life (Okay okay new love of the second) I had to choose this one.
Best icon Jojo has found (there are a few variations on this one):
and
Both made by dontburnhot on LJ.