Plug-in (computing), a piece of software which enhances another software application and usually cannot be run independently
Browser extension, which modifies the interface and/or behavior of web browsers
Add-on (Mozilla), a piece of software that enhances and customizes Mozilla-based applications
Add-ons for Firefox (addons.mozilla.org), the official Mozilla Foundation web site which serves as a repository of add-ons for Mozilla-based applications
Peripheral, an optional computer hardware component that significantly supplements or enhances the functionality of the original unit
Video game accessory, a piece of hardware used in conjunction with a video game console for playing video games
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