Gears on WordPress.com
On WordPress.com it is used to store all images and other web page components from the admin area to the user’s PC, speeding up access and reducing unnecessary web traffic.
The speed increase is most noticeable when Internet is slow or on high latency and makes everybody’s blogging experience more enjoyable.
Gears work on a bunch of browsers. Not all of them (Opera is noticeably missing), but good enough.