While it's not quite the same as hiring your own cartoonist, Cartoonize does a pretty decent job converting pictures into cartoon versions for those times you want to customise an avatar image or otherwise "cartoonise" a picture.
Visit Cartoonize.net and input a picture (either by uploading it or plugging in an image URL). Click "Cartoonize Now" and wait for a moment, Cartoonize spits back an image that looks like it has been run through the paint and relief filters in an image editor. Some things, like the highlights in the dashing Harry Potter photo we selected for testing, trip it up. Photos with even lighting and lower contrast seem to work better.
Visit the link below to take it for a spin or, if you want to get hands on, check out this tutorial on achieving the same effect (with more control) using GIMP.