If you're a Google Apps user, and you're not already using Google Labs, you should check it out. If you're already using it, check it again, because they just release a few new features. Below are list of the ones that I find very useful. As always, keep up to date on new features on the Gmail blog: http://gmailblog.blogspot.com/
If you want to enable Google Lab within your Mail account, click on the "Settings" link at the top of the screen. Look for the tab called "Labs" on the far right, and you'll see a screen like the one below. Lastly, there's a nice disclaimer on the features:
If (when) a Labs feature breaks, and you're having trouble getting into your account, there's an escape hatch -- just go to http://mail.google.com/a/your-domain-here.com/?labs=0 and Labs will be temporarily disabled.