The new generation of overclockers may not know, but overclocking is actually a very old practice. Even before the first Pentium processor, people were already squeezing more performance out of their hardware. Overclocking truely democratized with arrival of the Intel Celeron 300A which could be up-clocked from 300 MHz to 500 MHz without too much hassle. From the on forward, things went hard. We saw the Athlon XP-M CPUs clock up to 3GHz with a simple phase-change, and the Celeron D series hit over 8 GHz on liquid nitrogen.
In 2011 Intel changed their overclocking strategy and locked the bus frequency on Sandy Bridge CPUs. As the author of the Techspot article points out, even though people feared the end of overclocking we were given CPUs that could do sometimes 5 GHz stable on water cooling. Let us also not forget the wonders of AMD, who not only were the first to achieve 1000 MHz with their legendary Thunderbird CPUs, but were also (accidental) first to offer unlockable CPUs. That's right, with a bit of luck you could unlock a Phenom II X2 555 from two to four cores. Everything depending on your luck and karma level of course.
If you enjoy the occasional trip down memory lane, check out the article at Techspot.com here.