I haven't, but I would be interested.
I've joined a few swaps. One wasn't really for yarn but had received yarn in the package. Great way to try out other yarns you may not necessarily buy for yourself. I joined swap on Ravelry.
I have joined in other swaps, lots of fun.. but not yarn.. lets know what you come up with i could be interested :))
I would be interested in participating. I've done granny square swaps. Has anyone ever done a swap where an afghan is started and then sent to someone to do another row(s), etc?