Here are 5 most useful extension for magento

one step checkout make checkout process easy. In magento defualt checkout user waste more time to fill information in many steps but one step checkcut extension make it easy
http://www.magentocommerce.com/magento-connect/lotusbreath-one-step-checkout.html

2. Social Login for Magento
User can login with his google or facebook account it save time of registration process
http://www.magentocommerce.com/magento-connect/social-login-9.html

3. out-of-stock-notification
When any product out of stock customer can subscribe notification when product will be avaialable the customer receive an email. So that he can purchase it
http://www.magentocommerce.com/magento-connect/out-of-stock-notification-6.html

4. Magento-Wordpress Integration
By this extension we can show our wordpress blogs on magento site. sometimes we need to show blogs on our eCommerce site then this extension help
http://www.magentocommerce.com/magento-connect/wordpress-integration-3795.html

5. Fooman Google Analytics
Supports Google Universal Analytics (Classic Analytics are no longer supported)
Generate more sales through improved conversions! Create a comprehensive sales funnel by tracking every step of the checkout process (including sections retrieved via AJAX during one-page-checkout) - pinpoint precisely where potential customers are 'dropping off' during the checkout process
Optionally track to a secondary profile
http://www.magentocommerce.com/magento-connect/google-analytics-by-fooman.html