Gmail's huge success owes itself in big part to the widening range of applications, browser add-ons, styles, scripts and bookmarklets that work with it. From the get-go Google's stayed out of developers' way and turned a blind eye to unofficial Gmail add-ons, even ones that may very well violate its terms of service.

Source: http://lifehacker.com/software/lifehacker-...+ons-279556.php