These are the demo programs that accompany O'Reilly's "PNG: The Definitive Guide." rpng is a simple file viewer with the ability to set a background color when displaying transparent PNGs. rpng2 is a progressive PNG viewer, designed to work like a web browser (though it contains no network code); it can generate a background image when displaying transparent PNGs. wpng is a simple converter from NetPBM to PNG format; in most respects it is much more limited than pnmtopng, but it supports interactive entry of PNG text annotations and will generate noninterlaced PNGs on the fly (rather than buffering the entire image first like pnmtopng). The viewers include simple front ends for both X and Win32; the converter is a command-line app that compiles on Unix, Windows, and probably other platforms.
rsync is a replacement for rcp (and scp) that has many more features. It uses the "rsync algorithm" which provides a very fast method for remote files into sync. It does this by sending just the differences in the files across the link, without requiring that both sets of files are present at one of the ends of the link beforehand.
rsync-bc (rsync backup client) is a stripped-down version of rsync. The idea behind rsync-bc is to provide a simple client that only supports sending data. Receiving, local file transfers, and daemon/server modes have been disabled. No destructive file operation functions will be present in a binary.
Rsyncrypto allows you to encrypt a file or a directory structure such that they can later be synchronized to another machine using rsync. This means that local changes to the plain text file result in local changes to the cipher text file. rsyncrypto compresses the plain text file prior to encrypting it with gzip using the "rsyncable" patch, which is available from the rsync sources.