HashTab is a freeware OS extension that calculates file hashes. HashTab supports many hash algorithms such as MD5, SHA1, SHA2, RipeMD, HAVAL and Whirlpool (you can customize which hash values HashTab should compute). Hashtab is supported as a Windows shell extension and a Mac Finder plugin. When you right click and select properties you'll see a File Hashes tab in the properties window. When you click on the File Hashes tab HashTab will start to compute the file's hash values. You can then open the source file or just paste in the value of the original and HashTab will compare them.
Thanks to payday for the update.