2. Intel 8080 (1974)
Breakthrough application: MITS Altair 8800 (1975)
Some people would say that the 8080 was the first "real" microprocessor, the first one well suited to general-purpose computer use. This 8-bit CPU ignited the PC revolution as a part of the MITS Altair 8800, the first mass-produced personal-computer kit. The 8080's success quickly spawned competitors such as the Motorola 6800 and enhanced clones like the Z80, expanding the microprocessor's market potential. More important, the 8080 became the foundation of Intel's 80x dynasty of successors--a long, proud line that includes the 8085, the 8086, the 8088, and chips beyond. The 8080's footprint on history is quite large indeed.
Photos: CPU-World.com, Heinz Nixdorf Museumsforum