http://www.fermbyg.dk/Prohest/nix/img/Unix..._structures.gif
I like it, although it has some quirks ("balt" instead of "halt")
EDIT #1: Adding another one (not a visual one though, but still a cheat sheet), it's a hotlink to a PDF:
http://fosswire.com/wp-content/uploads/2007/08/fwunixref.pdf
EDIT #2: This article is reprinted from "The Official Ubuntu Book". While the cheat sheet is specific for Ubuntu Linux, most commands will work with other Linux distributions as well:
http://www.computerworld.com/action/articl...mp;pageNumber=1
EDIT #3: This one also provides links to online man pages:
http://www.debianadmin.com/basic-linux-com...-man-pages.html
EDIT #4: Ubuntu Quick Reference (hotlink to a PDF):
http://tux.fau.edu/index.php?action=dlatta...=111.0;attach=9
EDIT #5: 10 Linux commands you've never used - a must read in my opinion:
http://bashcurescancer.com/10-linux-comman...never-used.html
EDIT #6: This is a linux command line reference for common operations:
http://www.pixelbeat.org/cmdline.html


