Based on the I2C FAQ by Vince Himpe
In the early 1980's, NXP Semiconductors developed a simple bi-directional 2-wire bus for efficient inter-IC control. This bus is called the Inter-IC or I2C-bus. At present, NXP's IC range includes more than 150 CMOS and bipolar I2C-bus compatible types for performing communication functions between intelligent control devices (e.g. microcontrollers), general-purpose circuits (e.g. LCD drivers, remote I/O ports, memories) and application-oriented circuits (e.g. digital tuning and signal processing circuits for radio and video systems).
All I2C-bus compatible devices incorporate an on-chip interface which allows them to communicate directly with each other via the I2C-bus. This design concept solves the many interfacing problems encountered when designing digital control circuits. I2C has become a de facto world standard that is now implemented in over 1000 different ICs and is licensed to more than 50 companies.