Mac OS X contains many shortcuts, special keyboard combinations you can exercise to make things happen, but unless you make the effort to learn them there's no guarantee you'll recall the one you need at the time you most need it. Fear no more: there's ways to make finding shortcuts easier. Bring out your inner power user!
Dashboard is Apple's host application for small mini-apps, or widgets. It is a semi-transparent layer that's invisible unless activated.
There's all kinds of interesting Dashboard apps, including a series of handy little items which offer you useful lists of keyboard shortcuts, Dashkards.
Dashkards are cheat sheets you can add to your Mac OS X Dashboard and have them handy at the press of a button.
You'll find Dashcards for Mail, Pages, Safari, Yojimbo, OS X Text, Formatting, Webmail, trackpad and more. Take a look at the Dashcard siterthese free to downloand widgets which should help you find the right keyboard shortcut.
(Incidentally, I continue to think Dashboard could eventually become an environment in which you could run some iOS apps).
Unfortunately, there isn't a Dashcard for every application, that's where the next combination kicks in:
Master of command
There's so many keyboard commands -- how to remember them all? Fortunately, you don't have to.
You only need to remember one shortcut, a shortcut I'm going to share with you below.
Meanwhile you may want to take a look at LaunchBar (€24), a smart and powerful, keyboard-driven utility that lets you access and control applications, documents, contacts, calendars, bookmarks, media libraries, search engines and more.
The keyboard combination to rule them all, the heavyweight of the pack has to be Shift + CMD + /. What's it do? It invokes your application's help menu, giving you really easy access to Menu Items. But there's more...
Hover over a menu item and you'll be shown where it can be found along with a keyboard shortcut for the action. This may help you learn the command. As Adam Walters remarks, this is the keyboard combination to rule them all.
The other ruler
There is one more keyboard command that comes in incredibly useful, particularly when Flash has crashed your browser again: Force Quit (Command - Option - Escape). (The button with a curly pattern on it, the one beside it marked Alt/Option and the one marked esc at the top left of the keyboard).
This raises a menu which enables you to easily shut down applications which have become inactive. You can also achieve this via the Dock by clicking on the application icon while also pressing the Alt/Option key. You can also invoke this via the Apple menu at the top left of your Mac, and clicking 'Force Quit...'
Shortcuts -- the list
There's dozens of lists of shortcuts out there. Here's three:
Mac OS X keyboard shortcuts:
Apple's list of the most common shortcuts (or just use Shift + CMD + /)
Dan Rodney's List of Mac OS X Keyboard Shortcuts & Keystrokes
As it says on the can -- an extensive list which should be essential reading for any Mac user.
Relatively extensive collection of shortcuts grouped by application.