CommandLineFu helps you learn complex commands and check out their popularity via the votes on the page. This site also lets you easily copy the command straight from the site.
I have a printout of this one sitting next to my desk at work in case I'm having a horrible brain fart. This PDF lists several common commands and what they do for quick reference! This one is a must have if you're just starting out.
Check out O'Reilly.net for a huge listing of commands that you can also find in their book- Linux in a Netshell. This won't give you an example so to speak, but will help with confusing command descriptions.
This blog post from Free Video Lectures has links to 5 different sites that are extremely helpful for learning commands. Many of these list the information in a table of contents, and help you understand not only the command name, but also why it works.
On to the books! The Linux Command Line is a wonderful book if you've never even touched the OS. William Shotts describes everything in a way that's easy to understand and use to your advantage. I couldn't recommend this book enough, as it has helped me immensely.
Ubuntu for Non-Geeks makes using not only the terminal, but also the GUI easy. This is a great book to check out if you've only used strictly Windows growing up or if you're converting from MAC. Grant teaches how to do simple tasks like keeping Ubuntu updated to, of course, the terminal.
How could I not include the Linux Pocket Guide? Everyone I know has one of these, even Darren (though you'd think he has everything memorized!)
Working at the Ubuntu Command Line is a great nitty gritty book, and super cheap, to get into your more advanced tutorials quickly.
Ubuntu Made Easy is another great book by Grant, that helps you do all those menial tasks like setting up printers and using programs that are available in Linux. This is another one I recommend a ton!
You guys have sent in some great tips and feedback, keep 'em coming!. Thanks to everyone that has been sending feedback for our Linux Terminal 101 series. I've really enjoyed
learning what I have and I hope you enjoyed these resources.