Now all we need is a program that exports each frame of a certain character from a wad automatically so you don't have to do it manually.
If you mean sprites you could filter using Slade with the * wildcard character and then right click -> Gfx -> Export as PNG
If you mean a font definition (e.g., DBIGFONT), I think there was something made for that, let me know and I'll look for it.
EDIT: Thanks Boingo and Replica -- I forgot he probably wanted pngs!
Remember to convert the pngs from Indexed color to RGB in your favorite image editor if needed (GIMP, etc.)