Quick Base64 is a free and open-source application which encodes and decodes files into and from base64 format.
It is made to be fast, hence it's name (Quick Base64). It has a simple interface and uses drag and drop for most of it's commands.
It allows you to either encode a file to base64 or decode a base64 text to it's original file format.
Also, it is very reliable because of it's base64 detector which prevents you from decoding any ordinary text except text in base64 format. It has an auto-detect mode which automatically handles the encoding and decoding itself.
Source code can be found at Github