I saw this tip on the NGWList from Bob Jonkman. In the past, when I have wanted to decode a Base64 encoded part of a Mime message I have used websites like OpinionatedGeek but I saw a tip from Bob that makes life much easier, and it's able to do a lot more than just decode the text portion of a Mime. If you save out the Mime.822 with an extension of .b64 you can open it in Winzip to see the content.
So, for example, I had a mail in my mailbox that had a Base64 coded attachment - the attachment was actually a .png. I saved the Mime and cut out everything from it except the Base64 encoded part, and the begin and end Mime boundaries. I saved it as mime.b64 and opened it in WinZip - it displays the file in the WinZip interface and I can open it from there. Pretty cool - I certainly used to have to decode base64 message parts quite often when I was in support.