iComic is pretty well dead, development having ceased long before Snow Leopard. So I finally got off my arse to find a replacement.
I got back on my arse, because it was a symbolic gesture. We usually sit down to use the web. Anyway, I found
iStrip (not the publishing tool). It doesn't use external plugins, but has support for around 550 comics, and an adjustable concurrent download rate. If a strip won't load (currently Penny Arcade), it's usually due to the host site being redesigned, and will probably be fixed in the next release.