MiGBase64 is a very fast and small Base64 Codec written in Java. It doesn't create temporary arrays which many other codecs do. 7-100x faster than Sun's and 2-5x faster than Apache Commons codecs. Se homepage for performance charts.
Tiny, fast and effective, this Base64 implementation quickly became my choice for encoding and decoding within my project (net.sourceforge.easyml) !
The algorithm is detailed, yet elegant. The source comes with methods for input checking, as well as fast methods for when you know the data is legit. A great resource for anyone that wants to make sure they have Base64 functionality no matter what Java™ platform.