The links are mostly for Canon cameras, but there is a link for Nikon too: www.myshuttercount.com
For this one you just upload a photo, rather than installing a program on your memory card.
For Canon it looks like www.eoscount.com works well. They also have a FAQ page. Link here.
As an FYI, I will not assume any responsibility for you determining your count through these programs. You do so at your own risk. (Also I have yet to try one yet (no time), but I think I will go for the www.eoscount.com