For single strep/single operation - via plug-ins - there are a couple of choices:
1. Unplugged Set has a plug-in called "US Comic" (in effects from memory, or might be in tools). This reduces the image to a very blocky 4~8 colours (as in the 1950's comics, early posters, etc.).
... Unplugged (full set) is available (free) from this site - look in the free plug-ins areas or Harald's links.
2. If you are trying to get the more (colours) complex "Comic-look" effect, the best around (single step) is Amplifsoft Simplifier ... as mentioned in several of the earlier responses.
... The trick is in getting the sliders right.
For most comic-look conversions try this:
slider one ... (radius): value = 3
slider two .... (dark edge): value = 255
slider three .. (light edge): value = 24
slider four ... (spread): value = 1
... then apply. The settings will stay the same (unless you modify them) for the rest of the Photo-Shop or Paint Shop Pro session. They reset to default once you exit PS or PSP.
Tip. very slightly smooth or soften the image before applying Simplifier. It actually works better if edges are slightly feathered - rather than bold or enhanced.