Crappy unofficial adventure game merchandise is a dime a dozen on the internet, but I thought some of these Monkey Island t-shirts on Cafepress were pretty cool. I really like the giant monkey head design.
http://www.cafepress.com/mitees)
http://www.cafepress.com/mitees2)
Looks like they have these shirts available in more styles now.
When I was younger I remember seeing an Image in an American computer mag (I'm from the UK) for a t-shirt. It was the prize for a question only very small but the T-shirt had a cartoon picture of spit on it and said "It's a nice day for spitting".
I so wanted that and have never seen it since.