I found myself with a base64 encoded JPEG this afternoon that I needed to turn into a real live JPEG. Don’t ask how, I think it was a miscommunication between mail clients. I ended up turning to John Walker’s base64 utility for Linux.
That’s right, I downloaded the text attachment from my web-based client, trimmed the text to the raw base64 encoded portion, ftp’d it to a server, grabbed it from an SSH session on my Linux box at home, ran base64 -d attachment.txt outfile.jpg, ftp’d it up and down, and now everything is well.