pngcrush is an excellent batch-mode compression utility for PNG images. Depending on the application that created the original PNGs, it can improve the file size anywhere from a few percent to 40% or more (completely losslessly). The utility also allows specified PNG chunks (e.g. text comments) to be inserted or deleted, and it can fix incorrect gamma info written by Photoshop 5.0 as well as the erroneous iCCP chunk written by Photoshop 5.5.
pngquant is a batch conversion utility to quantize and dither truecolor PNG images, especially those with a full alpha channel, down to "RGBA-palette" PNGs (i.e., PLTE + tRNS in PNG parlance). Such images are usually two to four times smaller than the full 32-bit versions, and partial transparency is preserved quite nicely. This makes pngquant especially useful both for Web sites and for PlayStation 2 development, where one of the texture formats is RGBA-palette-based (though not PNG-compressed).
WebPack is a shell script for automatically packing Web sites by shrinking them without affecting their functionality or appearance. It is also useful for losslessly shrinking image collections and locating corrupt files. It works by stripping unnecessary information and optimizing the compression of images, and by removing comments/whitespace from HTML, using readily-available tools.
Versatile Encoder allows you to encode your own videos into good quality DivX files. The input can be a sequence of files (as generated by blender), a single file (if you want to recompress something), or a VCD (if you need to back up a movie). This piece of software tries to guess good defaults and to help the user in making choices.