Print Screen in Mac OS X 
Unlike Windows, there is no print screen button on the Apple keyboard. Depending on what you want to capture, you can choose one of the following methods to simulate a print screen.  

Capture the entire screen 
Press [Command] + [Shift] + [3] and release. 
Click any part of the screen with your mouse. 
The screen capture is stored on your desktop in .PNG format.  

Capture an application 
Open the application you want to capture on screen. 
Press [Command] + [Shift] + [4] and release. 
The cursor will change to a +. 
Press the [Spacebar] 
The cursor will change to a camera. 
Use the mouse (camera) to select the application. 
Check your desktop for the picture.  

Capture a portion of the screen 
Press [Command] + [Shift] + [4] and release. 
The cursor will change to a +. 
Drag your mouse (+) to select the area you want to capture . 
Check your desktop for the picture.  

TIP: To copy the image on your clipboard instead of automatically saving it on the desktop, hold the [Ctrl] key along with [Command] + [Shift] + [4].  

Show Hidden Files in Mac OS X 
Open the Terminal application. You can find it under Applications -> Utilities. Type defaults write com.apple.finder AppleShowAllFiles Yes. Launch the Finder to see the hidden files. They are pre-fixed with a dot(.). Please be careful while changing anything in these files. The OS X uses them to manage the Mac. 

TIP: To move to specific directory in the Terminal, type “cd” and drag the directory from the Finder to the terminal and hit enter. You can now access the files in this directory using the Terminal 

Useful shortcuts
Use [Command] + [Space] to open the Spotlight search bar. 
Use [Command] + [Option] + [Del] (Delete) to empty trash. 
Use [Command] + [Q] to quit an application. 
Use [Command] + [Option] + [Ctrl] + [8] to invert the colors on your Mac. 

Dock Tips 
Add an already open application to the dock: Press [Ctrl] + mouse click. From the context menu, select “Keep in Dock” 
Add a closed application to the dock: Drag it from the Finder to the Dock. From the context menu, select “Keep in Dock”
Remove an application: Drag the item off the dock and it will disappear in the cloud of smoke From the context menu, select “Keep in Dock”
Resizing an application: Place the cursor over the vertical separator bar and right click the mouse.

Miscellaneous Tips 

1. To mute the feedback (blip sound) that the Mac makes when you turn up or turn down the volume using the volume keys. Hold [Shift] when you turn up or turn down the volume. You can permanently turn it off by going to System Preferences > Sound and un-checking the “Play feedback when volume is changed” option. 
2. While browsing the web using Safari, want to know the meaning of a word you just read? If you select the word and hit [Ctrl] + [Command] + [D], a pop up window will give you the dictionary description of that word. If you move your cursor to another word, the description for that word will show up, too. 
3. You want to save a piece of information that you came across while browsing? Select the piece of information and press [Command] + [Shift] + [Y]. Mac will create a new sticky note and save the information in it.
4. Switching Between Windows: Use [Command] + [Tab] to switch between different application windows that you have opened. You can use your mouse to click the exact application you want, or keep hitting the TAB till you sequentially get to the one you want. You can also use the arrow keys while you hold down the command button to move back and forth between the applications listed. Use [Command] + [~] (tilde) to switch between the windows of the same application.
5. While booting up the Mac: Hold [C] down to boot from a CD or DVD. Hold [D] down to boot the disk’s first partition. Hold [N] down to boot from a network server. Hold [Shift] to boot into safe mode. Hold [Command] + [S] to boot into a single user mode. Hold [Option] to choose startup disk at boot time

Automator Basics
The Apple Automator application allows Mac OS users to automate the repetitive computer tasks by creating a workflow. A workflow is a series of actions that will be executed one after the other when executed. An action is the simplest unit of the Automator like Copy to Clipboard By combining actions from different applications in the drag and drop interface of the Automator, you can create your own applications and plug ins. Here is how it works. 

1. Open Automator.app from the Applications folder.
2. You will see 4 panes: Library, Action and Help on the left; the workflow canvas on the right.
3. You can search for actions by typing in the edit box located on the top left corner of the application.

Create a plug in for sending mail from the Finder  
 

1. Press [Command] + [N] to create a new workflow.  
2. Select Finder in the Library list.  
3. Select Get Selected Finder items.  
4. Drag it to the workflow canvas on the right.  
5. Now select Mail application in the Library List.  
6. Select Get New Mail.  
7. Drag it to the right window, below the Get Selected Finder Items box.  
8. Select the Add Attachment to Front Message from the Action pane and drop it as the last action on the workflow.  
9. You have just completed your workflow.  
10. Click the Run button on the top left corner of the screen.  
11. The Mail app will pop up. Close it and go back to the Automator application  
12. Open File > Save as a Plug-in. Type a name for the Automator plug-in.  
13. Open the Finder window and click on a file that you would like to send as an email attachment.  
14. From the context menu([Ctrl] + click), select Automator and then choose the workflow you just created.  
15. If you have your mail account set-up, you just need to type in the details of who you want to send the message to. You can now send attachments directly from your Finder window.  

TIP: You can also save the Automator workflow as application using File > Save As. Choose “Application” in the File Format dropdown.
You can then double-click on the application to launch it later, instead of using it directly from the Finder.
If you want to save your workflow to edit later, you can choose to save it in the nascent workflow format. Under the File > Save as dialog box, choose workflow as the file format.