痛車 (pron: "Ita-sha") are cars whose exteriors have been modified to an anime any other two dimesional moe~ theme. The term comes from normal people making fun of otaku's cars in disgust by coining them to be an 痛い車 (pron: "itai-kuruma" lit trans: cars that make people say "ouch" in disgust).
"itai-kuruma" then became abbreviated to "ita-sha" (痛車). This abbreviation is some what intentional as it is cynically a homonym for an "Italian car" (イタ車). Italian cars (イタ車) are known to be liberal and intriguing in design, and so are Ita-sha (痛車)....more or less in the wrong way.
Rather than trying to propogate the term "ita-sha" into the English otaku masses, I'll use the literal translation "ouch-car" onward.
One can find a whole parking lot full of said ouch-cars at most anime type events around Japan. And since Comike is the largest gathering of otakus in Japan, one can expect to find many ouch-cars parked outside. While it may not be as cool as seeing souped up cars or duels of which car has the largest bass from their sub-woofers, one can find a mini-car show of "who has the most ouchest-car" in the lot. wwwww
Well I know all of you like visual references so here are some from youtube (or feel free to search with the term "痛車" at jp.youtube.com):
Good how-to video if you are interested in making your own ouch-car:
Some ouch-cars from Comike 71: