1. Unix Tool Box: An incredibly exhaustive reference for all things Linux. This document is a collection of Unix/Linux/BSD commands and tasks which are useful for IT work or for advanced users.
2. One page Linux Manual: Great one page reference to the most popular Linux commands, it is a summary of useful Linux commands.
3. Linux Reference Card: One great reference published by FOSSwire.
4. Linux Command Line Cheat Sheet: This is an interestingly sorted and helpful cheat sheet by cheatography.
5. Linux Command Line Tips: This is a linux command line reference for common operations. Cleanly sorted and well described.
6. Treebeard’s Unix Cheat Sheet: A great reference that shows command comparisons with that of DOS. So if you are someone who was a DOS user and has switched to Linux, this is the best one too have!
7. Linux Shortcuts and Commands: This is a practical selection of the commands we use most often.