I created an Applescript droplet for verifying the 'NSA Secure Hash Algorithm 1' checksum that is sometimes supplied to authenticate a download. It's pretty simple, calling a shell script to output the checksum hash.
Just paste the following into AppleScript Editor and save the script as an Application.
-- if applet is double-clicked
set chosenFile to choose file with prompt "Select the file to checksum:"
do shell script "/usr/bin/openssl sha1 " & quoted form of POSIX path of chosenFile
display dialog result buttons {"OK"} default button "OK"
-- or if the file is drag and dropped
on open chosenFile
do shell script "/usr/bin/openssl sha1" & quoted form of POSIX path of chosenFile
display dialog result buttons {"OK"} default button "OK"
end open
You can either drag the file to verify onto the droplet, or you can double-click the applet and navigate to the file in question. Either approach should yield a dialog with the file's path and its resulting checksum hash.
The code was assembled from multiple sources, including Apple's Developer site, and tested by me under Snow Leopard.
[crarko adds: I tested this, and it works as described. This earlier hint describes a command-line tool (md5deep) that can be used to calculate a variety of checksums.]

