If you want to download a program, the following restrictions will apply unless stated otherwise:
Windows programs run on Windows 2000, XP, or Vista.
Mac OSX programs are universal binaries and run on version 10.2 or later.
Linux programs are officially supported on Ubuntu 6.10 or later, SUSE Linux Enterprise Desktop 10, and Red Hat Enterprise Linux 5. They may work on other flavors of Linux as well; you'll just have to try them and see.
Linux programs also need: x86-based Linux distributions with GTK+ 2.8 (or higher), glibc-2.4, CUPS (Common UNIX Printing System), and libstdc++.so.6.
