W-Packager is a packager similar to dpkg that can be used by anyone to create and maintain Debian packages under Linux or other Unix systems. The aim of the project is primarily to have a workable version of a packager that can be compiled on many systems, including those that do not support fork(). At this time, W-Packager is used within UniGW. It can also be used under Linux.
slkbuild is a script inspired by makepkg from Arch that greatly simplifies the package building process in Slackware and derivatives. It parses an easy-to-create SLKBUILD meta-file and from that creates a conventional build script that follows all of the Slackware packaging standards and that can be run on its own. The advantage of using slkbuild is that it ensures package uniformity and allows for an easy-to-edit meta-file in the event that one might require customization or might need to update the script for new releases. It also makes the build process much quicker, since it takes care of downloading the source, untarring, gzipping man and info pages, stripping binaries, making sure that the menu entry for a graphical application is compliant, as well as a host of other things.
pkg++ is a tool to build packages. The goal is to provide a tool you can use to build a package from a generic recipe on your distribution/OS and for your native package manager. The recipes, when done correctly, can be used on multiple OSes, and thus users can share them even if they do not use the same distribution. It is as much a way to easily package as it is a way to create multi-OS package recipes repositories.