Internet services, Skype included, have turned phone calls into cheap software add-ons, not stand-alone products.

Some software add-ons provide operability with specific hardware, such as game controllers and optical motion sensors.

A predecessor to the cartridge slots on later Commodore machines, they allowed various software add-ons such as machine language monitors.

Object-Oriented Programming design allows great flexibility for licensees to develop cross-platform software add-ons.

Introduced in 1989, Xtensions, along with Apple Computer's Hypercard, were among the first examples of a developer allowing others to create software add-ons for their application.

Since then it has slimmed down and increased its capabilities, in part through software add-ons that link it to other applications where digital writing may come in handy.

The trouble with Windows 2000, of course, is that it is incompatible with many hardware and software add-ons, including such essential titles as Tomb Raider and Barney the Dinosaur CD's.