If your using windows, you probably know what a pain it is to calculate the MD5 sum of file you just downloaded.   Sure you can track down a command line version of md5sum, or install Cygwin, but I’ve found something handier.
HashCheck Shell Extension is a Windows Explorer extension that adds a new tab to the file properties dialog.  The new tab, when access calculate the md5 and long with most other checksums of the file in question.
You can find screenshots and the download at http://code.kliu.org/hashcheck/.