GMail Labs provides some of the following features which are very useful
- Recall a message in GMail
- Make reply to all as default option
- Mark as Read Button
- Custom Keyboard Shortcuts
- Number of unread emails
- Preview Email Message
- Enable Preview Pane
- Change the default Inbox Type
GMail labs is by default disabled for Google Apps Users but you can enable this feature using Google Apps Email Settings.
Login to Google Apps account as an administrator then click on the Settings tab. In the Setting screen, navigate to Email Settings page by clicking on Email link available under the Services tab.
Scroll down to GMail Labs section and mark the check box with label as Enable Gmail Labs for my users. Then click the Save changes button available at the bottom of the page. By this way you can enable Gmail lab features for your Google Apps Users.