Just because lineart was mentioned before, a little comment from me: one of my friends told me the way he cleans up lineart in photoshop...he ups the picture to an enormous size - 2700 by 2745 pixels is the example size I'm using, and I think he's gone even larger before - and then uses the polygonal lasso tool (I think that's what he said) to go over the outlines somehow...
Not exactly sure if it works every time, since I've never tried it myself, but it does work somewhat well; he made some lineart of mine look much better. :P Then again, the paper I'd inked on wasn't the best, and my pen was dying...
Of course, it makes for a huge file if it's left at that size uncompressed. :/