The Share This button, originally a WordPress plugin by Alex King (interview), is now a separate project, along with the Share This icon and everything. It’s hosted these days, but you can still download the self-hosted plugin should you prefer not relying on an external service.
There it is. Cool.
They use a special layer when clicked, rather than the default ones, although they still stick to the basic style of the plugin. Click it, or see below:
Share This have spread tremendously, at least the icon can be seen on newspapers as well as tech industry leading blogs. Heck, even Google uses it for their share services! Branding gone well, I’d say.