Digital Invisible Ink Toolkit
The Digital Invisible Ink Toolkit is a Java steganography tool that can hide any sort of file inside a digital image (regarding that the message will fit, and the image is 24 bit colour). It will work on Windows, Linux and Mac OS because it is written in Java and thus platform independent. There are four highly customisable algorithms in the tool, as well as an open-source implementation of RS Analysis (an extremely good steganalysis method). The tool has the additional advantage of being able to simulate hiding - so you can get an accurate map of where the information is hidden.