I'll start with an iconic one:

Houck's Panacea, Baltimore, MD.  Counterstamp on an 1834 O-102 Bust Half Dollar.  Brunk H-779, HT-141.  Choice Very Fine with pleasant deep gray toning over all surfaces and hints of more colorful highlights at the peripheries.  Very well struck for the issue.

I found this pic online not too long ago (sorry, I couldn't resist; had to share it!) Not sure what company slabbed it, but nevertheless, it's been aggressively "CAC'd!"

Alright, it has been over two weeks without a post on this thread.  I guess there aren't many counterstamp collectors out there...?  So, I'll throw another one out there for your consideration/comments:

"A.G." counterstamped within a wide-toothed rectangular depression on a U.S. 1807 Large Cent, HT-A458, R9, Unique, Ex-Dr. Sol Taylor collection, Brunk plate coin.  Brunk states this isn't a mark of a silversmith, pewterer, or gunsmith.  The stamp remains unattributed but is assigned to the Hard Times era by Brunk.

nice! I've always wanted a counter stamp! here's one I found on Google!

have you guys read this month's "Numismatist?" they have a really cool article on a particular counter stamp!

I saw that one as well ... nice article.  I am personally more into merchant counterstamps, but I wouldn't turn that one down if offered to me!  :D

Here is a counterpunched Ike which commemorates the Bicentennial, as well as the Centennial of the Statue of Liberty.

Looking for some more counterstamps out there!  Here's one that might energize you. 

J.F. McKenney worked with his father in Biddeford, Maine in the 1850s.  He later established his own shops in Bath and Saco.  This is a popular and rather uncommon stamp which can be found in differing variations.  This example is well-stamped, with the "J.F." typically seen on most examples missing from this one.  The host coin is a pleasing, dark walnut color. 

(1849-1850) Biddeford Maine on 1848 1C, McKENNEY GUNSMITHS, Brunk M-476, Rulau ME-2D, Ex-John J. Ford and F.C.C. Boyd

 

This counterstamped Maria Theresa Thaler is valued more in Europe than it is in the US.  I have seen examples sell for as high as€400.00 (US$ 500) however the mode is  is much closer to €250(US$300.00). The purpose is pretty self explanatory. In November 1963 500 MTT were purchased new from the Vienna mint.  457 were countermarked.  The coins were issued to attendees at a celebratory Gala in March 1964. Members who didn't attend had to pick it up in person...many didn't.   The membership at that time was 150 members. It wasn't an overly successful issue and around 200 of the coins were sold to  a Florida Coin dealer. The dealer responding to enquiries in 1968  reported 90% of the coins were unsold.  It is believed, not confirmed,  that at the height of the Hunt brothers Silver machinations that dealer sold the remainder to a refinery for the silver value.  I think this issue is an undervalued piece of American Numismatic History

Neat counterstamp Ian!  And on a beautiful coin too.  Thanks for posting it.  I'm going to have to keep my eye out for that one.  :)