The Base64 Content-Transfer-Encoding is designed to represent arbitrary sequences of octets in a form that need not be humanly readable. The encoding and decoding algorithms are simple, but the encoded data are consistently only about 33 percent larger than the unencoded data.
Base64 encoder and decoder online
Base64 decoder, decrypt - simple online form for base64 data, decrypt the data
Base64 encoder, encrypt - the same to encode text data
Other resources for Base64
Base64 on Wiki - description of Base64 algorithm
Rfc 3548 - The Base16, Base32, and Base64 Data Encodings