Alien is a program that converts between the rpm, dpkg, stampede slp, and slackware tgz file formats. If you want to use a package from another distribution than the one you have installed on your system, you can use alien to convert it to your preferred package format and install it.
Despite the large version number, alien is still (and will probably always be) rather experimental software. It has been used and tested for over a year now but there are still many bugs and limitations.
Alien should not be used to replace important system packages, like sysvinit, shared libraries, or other things that are essential for the functioning of your system. Many of these packages are set up differently by Debian and Red Hat, and packages from the different distributions cannot be used interchangably. In general, if you can't uninstall the package without breaking your system, don't try to replace it with an alien version.
Alien has successfully been used for converting add on software, such as Applixware, Metro X, and many other packages.
To use alien, you will need several other programs. Alien is a perl program, and requires perl version 5.004 or greater.
To convert packages to or from rpms, you need the Red Hat Package Manager; get it from its web site.
If you want to convert packages into debian packages, you will need the dpkg, dpkg-dev, and debmake packages, which are available on the Debian ftp site. You'll also need gcc, and make.
Attention, all linux users who don't use Debian: Bruce S. Babcock has put together a package of all the extra files you need to use alien on a non-Debian distribution. It's called "alien-extra", and you can download it from his ftp site.
Solaris packages of the software you need to use alien are also available, thanks to Mark A. Hershberger.
I welcome all bug reports, feature requests, and comments. Just email me.
Also of interest is my comparison of features of the package formats alien supports.