Add-on

Software

 * 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
 * Expansion pack, an add-on for a video game

Hardware

 * Peripheral, an optional computer hardware component that 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

Other

 * An extension to a house
 * Addon, a person named in the Hebrew bible
 * An adjuvant therapy, an add-on therapy to existing treatment