"There's a web app for that" is a Chrome extension that suggests Chrome Web Store apps that you may want to install. It has two UIs:
- A page action (a blue [+] icon) shows up on sites that have a corresponding Chrome Web Store entry.
- The options page (shown when the extension is first installed) shows sites from your browser history that have a Chrome Web Store entry.
The extension can be found in the Chrome Web Store.
Matching up sites and Chrome Web Store entries
The extension includes a dump of the top ~1000 web store apps (see
extension/app-data.js). For each app there is a set of URL patterns that it should match. This data is derived from the
urls section in the apps' manifest. This means that no personal data is sent to the Chrome Web Store (or any other server) to see if a URL correponds to a store entry.