How to convert between packages (deb, rpm, tgz) using alien on Linux

alien is a program that converts between Redhat rpm, Debian deb, Stampede slp, Slackware tgz, and Solaris pkg file formats. If you want to use a package from another linux 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. It also supports LSB packages

Warning:
Alien should not be used to replace important system packages, like init, libc, or other things that are essential for the functioning of your system. Many of these packages are set up differently by the different distributions, and packages from the different distributions cannot be used interchangeably. In general, if you can't remove a package without breaking your system, don't try to replace it with an alien version.

Installation:
Ubuntu user can install alien using following command: apt-get install alien
OpenSuSe user can install alien using "1-click" installer - here
Fedora user can install alien using command: yum install alien

Examples
Convert the package.rpm into a package.deb
alien --to-deb package.rpm

Convert the package.deb into a package.rpm
alien --to-rpm package.deb

Convert the package.rpm into a package.deb (converting to a .deb package is default, so you need not specify --to-deb), and install the generated package.
alien -i package.rpm

alien --to-deb --to-rpm --to-tgz --to-slp foo.deb bar.rpm baz.tgz
Creates 9 new packages. When it is done, foo bar and baz are available in all 4 package formats.