This is an interesting one I found on Shell Fu this morning. It’s similar to my previous post of hiding a .exe in a text file. They both have there uses.
It is possible to hide a rar archive inside a png image file and then retrieve the files from the image.
cat picture.png archive.rar > hidden_archive_in_pic.png
This can also be done on Windows:
copy picture.png + archive.rar hidden_archive_in_pic.png
When you want to retrieve the hidden files, download the image, rename to .rar and extract.