In the world there's a lot of programs that can run in many unix platforms. A big share of thease is free and open source.
Very important is the software written by GNU organization with the aim to develop a complete Unix-like operating system, free and open source.
Very important is OSI (OpenSourceIniziative) too.
For Unix there's a lot of business software too, then who work in Unix environment doesn't feel homesick for Microsoft environment.
Unix environment are more stable and speed than Windows.

The GNU project is borne in 1984 writing new software and using free software written by others.
The founder and leader is Richard Stallman.
Important software written by GNU are the gcc compiler, the emacs editor, gtar, gzip, bash and others.
If GNU wasn't been, linux could not to bear.
Perhaps is the GPL license the key of the success of the free software becouse it encourages others programmer in sharing software.
The kernel Linux too is a GPL software!

The OSI (Open Source Iniziative) is newer but have already big success as mozilla (netscape) and Open Office (Star Office).
The OSI leader is Eric Raymond, the author of "The cathedral and the bazaar".