There is a collectable towel (yes, a towel) commemorating the late
Douglas Adams. From what I've heard, it is only available from
England, or perhaps other parts of the UK. It has the number "42"
enscribed in it, among other things. I don't think it's that rare,
I've just been unable to locate a retailer who sells it. Basically,
I'm looking for any retailer who sells this towel. :)

According to the H2G2 site at the BBC, there is a Hiker's Towel
Comapny at XTV. Whether that is a real company or not I don't know.

You can get to the H2G2 site from:

http://www.douglasadams.com

hlabadie-ga

Hmm, guess this is going to expire :) Though I probably didn't offer enough.
Just in case, though, here's a little more info.

http://homepage.mac.com/kentfield/dna.htm

It's the first pic, and quoted from the guy who owns it:

I have it on good authority (from the one that I purchased said towel 
from) that the original towels were produced in 'blue text on white' 
(which I have) and 'white on beeblebrox brown' (one of which I have seen 
  before in the flesh many years ago being used as a towel and stupidly 
let slip through my paws). There were approx. 5000 originally produced 
at the request of Beer Davis Publicity (who managed all the Hitchhiker 
publicity)....

MJ - feel free to butt in and correct any of this

....and the towel that I have is the orignal M&S prototype. The actual 
production run was by another company, as M&S didn't think there would 
be a market for it (doh!). The only difference between the prototype and 
the production run is that the prototype has a page number on it which 
the production run towels do not.

Heck, I'll tip well if it's actually found...I just didn't want to
offer any more and have the wrong towel be located :)