It’s amazing how long it took me to find this one, me an avid comics reader…
iComic is an off-line comics reader which grabs the strips while you’re on-line and then let’s you read them when you are off-line.
It has a nice set of features, the nicest of which is that it incorporates new comics via a perl plugin architecture.
I haven’t had the need to write any plugin yet as all my usual comics are already available either with plugins from the iComics own site or with plugins gathered from other sources (the plugin for User Friendly, for example, was available in Version Tracker).