To find them, click the settings wheel in the top-right of your Gmail window and scroll down to "Get Add-Ons." Easy. There are ten plugins available right now (which you can browse here), including ones that integrate emails into Asana and Trello tasks as well as Intuit Quickbooks and Smartsheet.
Sure, the initial slew is heavily business-centric, but Google noted in March that it was already working with several companies for work-centric integrations. In the blog post announcing the availability of add-ons, Google stated: "If you're a developer, you can also easily create add-ons for your app or your organization—write your add-on code once and it will run natively in Gmail on web and Android right away."
Unfortunately, it seems the add-ons are only live in browsers and on Android devices at the moment, with no clear date when they will head to other platforms. When reached for comment, Google spokesperson Brooks Hocog said: "We're working with Apple to bring Gmail Add-ons to iOS users."