www.whitsoftdev created many years ago (lol I've been using it off and on for YEARS and recently refound it) a very simple md5 checksum utility. http://www.whitsoftdev.com/md5/ i didnt see it listed, but from jogging my memory was able to "refind" it. It seems "portable" to me, but didn't do extensive testing, it simply prints out a checksum of a file that you point the app to from it's GUI.
It does have support for context menu and the "Send to >" menu within Explorer which is what I like the most, to compare a file checksum really quick.