Add-ons are optional computer hardware or software modules that supplement or enhance the original unit they are adding on to. Also known as plugins, extensions, snap-ins, or various other vendor-specific names, these modules often have proprietary interfaces that are tightly held by the company that manufactured the original unit in order to prevent competing companies from producing add-ons.

For example, a network switch may ship with an unoccupied but non-standard port to accommodate various optional physical layer connectors, while games and productivity applications often use plug-in architectures which allow original and third-party publishers to add functionality. Web browsers use plug-ins to enable the presentation of new content formats without modifying the underlying web browser.

Manufacturers can use add-ons can to create vendor lock-in by limiting upgrade options to only those available from or endorsed by the original manufacturer. IBM's Micro Channel architecture, technically superior to Industry Standard Architecture as a way to add components to IBM PCs, largely failed to gain wide support due to the difficulty in getting certification for third-party devices.