Linux: If you've been itching to try Linux but are intimidated by all the command-line work, free download CLIcompanion will ease your fears, providing a customisable list of popular commands and what they do.
CLIcompanion is essentially a terminal with a searchable dictionary attached. Thus, if you want to do something, you can just search for the command (or for words in its description) and see a description of the command, along with any useful flags it may come with. Then you can just execute the command right there in the attached terminal.
The pre-packaged list is fairly short, though it'll certainly get you started. You can, however, add new commands and descriptions at any time, so as you sift through online forums and the Terminal's man pages (e.g. typing
man ls will give you a detailed manual for the
ls command), you can add those commands to your dictionary for later reference.
CLIcompanion is a free download for Linux only.