In one group of friends, we've had a collaboration going for ...well, years now. It's silly, basically just about coming up with word plays, puns, silly names - playing with language in general.
Some of it has morphed into silly t-shirt designs. We might be setting up a webshop one of these days, but for now they are mostly just for our own consumption.
I know it might seem like we have too much time on our hands :)