Run command in terminal, defaults write com.apple.Finder AppleShowAllFiles YES

Open System Preferences and click Sharing. Click Firewall, and then click Start. To block incoming traffic on ports used by one of the sharing services, disable that service in the Services pane.

From the Finder's Go menu, choose Utilities. Open Directory Utility. Click the lock in the Directory Utility window. Enter an administrator account name and password, then click OK. Choose Enable Root User from the Edit menu. Enter the root password you wish to use in both the Password and Verify fields, then click OK.

From the Finder's Go menu, choose Utilities. Open Directory Utility. Click the lock in the Directory Utility window. Enter an administrator account name and password, then click OK. Choose Enable Root User from the Edit menu. Enter the root password you wish to use in both the Password and Verify fields, then click OK. Open System Preferences and click Sharing. Click Firewall, and then click Start. To block incoming traffic on ports used by one of the sharing services, disable that service in the Services pane.

Open Safari, goto Safari > Preferences. Select Appearence and uncheck Display images when the page opens

To take a screen shot of entire screen press Command + Shift Keys +3
Press Command + Shift keys + 4, cursor will cahneg into crosshairs, now you can use to select whatever portion of your display you’d like to capture in a screen shot.
If you hit the Spacebar after typing Command + Shift key + 4, cursor will replace by camera. With camera, you can take a screen shot of the Dock, the entire menu bar, a single open menu, the desktop, or any open window.

Select Dock from Apple menu and click on Dock Preferences, a dialog will open giving you access to all of the Dock customization options.You can change size of the Dock, select the location where the Dock should appear, Genie/scale effect, Bounce effect.

Open the Applications folder, select the application and drag it to the Dock.

Goto Apple menu, select System Preferences > Desktop & Screen Saver and click on Screen Saver tab. Left column lists available screen savers.

In Mac OS if you are running lot of windows, you can bring all windows in front by using Expose. Press the F9 key and Expose instantly creates thumbnails of the open windows and displays them on screen. Click on the desire window you want and it will switch the application.
You can press the F10 key to create thumbnails of the open windows of your current application. Or F11 to move all open windows to the side, so you can see the files on your desktop.

Restrict your web pages to open in new Tab instead of new window run following command in Terminal
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true
Restart Safari to take effect.
To revert your setting run command
defaults write com.apple.Safari TargetedClicksCreateTabs -bool false

To remove third party plugins, make sure Mail is not running, close your Mail if it is running. Go to folder /Library/Mail/Bundles folder. Delete all the files/folders from this folder. Go to folder /Library/Mail/Bundles. Delete all the contents of the folder. Now open your Mail, it should work fine.

To Enable password protection, go to System Preferences > Security > General tab and check option Require password to wake this computer from sleep or screen saver. By enabling this, it will prompt for password when ever machine is wake up from sleep or screen saver.

To enable fast user switching, goto System Preferences > Accounts > Login Options , click on Enble fast user switcihing checkbox. To switch, from the menu bar click on right side where the logged in user name appears. A drop down menu will appear, click on Login Window. This will open login screen, select the user you want to login/switch.

If you are unable to quit an application in Mac, then try to Force Quit the application. From Apple Menu select Force Quit and select the application and click the Force Quit button. Also, you can use Cmd-Option-Esc sequence to pop this menu.

To get additional information for directories, open Finder > Macintosh HD >Command + J. This will open properties window. Check the option Show Item info. Now, you can get information like the number of items in a directory, the dimensions of an image, or the length of a music or video file.

To enable/retrieve Macintosh HD icon on desktop, click on Desktop, press Command+J, enable the option Show hard Drives.

Go to System Preferences > Dock, check the option Minimize windows into application icon. This will minimize window to their assigned application icon. Right-click the icon to select from multiple open windows.
You can also run this using terminal, run defaults write com.apple.dock minimize-to-application -BOOL YES in terminal window.

Mac OS X keeps all the captured screenshots at desktop, by default but you can change it to your favorite location. From Terminal, execute the command defaults write com.apple.screencapture location /your location. After execution run another command to refresh the changeskillall SystemUIServer.

To hide the dock in Mac, press Alt+Cmd+D i.e. Option+Command+D combination. Press the same combination to get the dock back.