Safari Extensions are small programs or plugins for Safari web browser that are very useful for users, because they add many interesting features and functions their web browser.
These extensions can be easily installed directly from Apple's Safari store, accessible through the Menu (cogwheel icon) → Safari Extensions... option.
Here is a list of some really useful extensions for Safari web browser.
Safari Social Extensions
Twitter for Safari
The official Twitter extension for Safari brings real-time trends and information to every web page. Users can keep track of what’s trending at the moment, including any mention of the web page they are viewing.
Better Facebook improves Facebook experience. Users can customize their news feed with advanced filters that can hide already read posts, alert about new comments and more.
Stop constantly checking your Google+ tab to see if anything new has been posted. LetGooglePlus do it for you. This extension adds a toolbar button giving you the current number of unread Google+ notifications waiting for your attention.
Click the Thurly button in the Safari toolbar to shorten the URL of the web page you’re currently viewing. Then copy and paste to share the URL on your go-to social networking sites, in an email or a chat, or anywhere else on the web.
Safari Security Extensions
AdBlock is a useful Safari Extensions that blocks all ads on all web pages, once it is installed.
Keep all your logins completely in sync. No more worries about remembering your passwords. Access your favorite sites quickly and securely.
LastPass is an online password manager and form filler that makes web browsing easier and more secure.
Safari Utility Extensions
Awesome Screenshot is very handy Safari Extension for capturing, annotation, and sharing of screenshots with just a click. Many options like crop or blur are available to alter the screenshot. Whole web pages can be captured as well.
Translate adds a toolbar item that translates the current web page using Google Translate. Very useful for people who often translate whole pages.
My eBay Manager
The My eBay Manager extension for Safari helps users keep track of their eBay activity while browsing on internet. Uses are always securely signed in, so they can check their progress whenever they like.
Add a weather button to the toolbar. The Cloudy extension can show the current temperature as a badge on the button, and can open Yahoo! Weather.
Google Reader Tools
Customize Google Reader with the built-in Helvetireader or Lucidia styles, or use your own stylesheet. A toolbar button allows you to check your Google Reader account for unread items.
This useful Safari extension automatically restores previous Safari browsing session.
Safari Developer Extensions
Firebug Lite for Safari
The Validator Safari extension checks the validity of (X)HTML code, CSS styles, and RSS feed using W3C Validators and optionally displays the count of errors and warnings found in the badge.