Used stamps exchanges.

In Canada, they probably no longer have seals. :-) I also received from Canada a letter canceled with a pen recently.

Australia Post do the same thing.

Of course, it's hard to find out who did it. :-)

US Post does the same with covers - "artistic" pen cancels :(

I have to put my letters in a street box, I usually con't make it to the actual post office, so not sure where they go from there for cancellation.