HashTab provides OS extensions to calculate file hashes. HashTab supports many hash algorithms such as MD5, SHA1, SHA2, RipeMD, HAVAL and Whirlpool. Hashtab is supported as a Windows shell extension and a Mac Finder plugin. HashTab provides an easy way to verify file integrity and authenticity.
Once you have installed HashTab, just right click on any file. On Windows, select properties and you will see a new "File Hashes" tab. On Mac select "File Hashes" (on 10.5 the "File Hashes" menu item will be found under the "More" submenu). This is will start HashTab providing more or less the same view that you see on Windows. This File Hashes window displays all the hashes for the file. You can customize which hashes are calculated and displayed. You can hash other files for comparison. You can also paste in hash text so you don't go cross-eyed trying to compare MD5 hashes. See below for more ways to use Hashtab.
For more information and screenshots on how to use HashTab on OSX see the HashTab Mac page.