Hawk Wings lists 202 different Stamp icons 
for Mail.app, all free.
I’ve just noticed, though, that it is possible to buy Mail Stamp icons.
The Wacky Pixel company offers some free ones, but also sells complete sets in an online store.
You can pick up a set of 28 musical instrument stamps for USD 3.95, 40 African Safari icons for USD 4.50 or 40 WWII air combat icons for USD 4.50:
Cheaper than real stamps!
You can also order custom icons online using your own images, one for USD 5.50 and three for USD 9.75.
But… why?
But… why not?
Interface hacks big and small are part of the fun.
They are more of a fad among Windows users, perhaps, whose need is greater, but fiddling around with the GUI is a time-honoured geeky activity.
It’s not for everyone, but it’s good that there are resources around for the people who like to tweak.