Now you're making me nostalgic. 10-15 years ago was both my teenage years and the end of the overclocking golden era. I remember overclocking the living shit out of all kinds of Core2 stuff without anything except a stock cooler and a modest voltage bump that was still within Intel's "safe" recommendations.
One of the best gains was with an E5200 I spend around $120 on brand new. Bumped that up from 2.5GHz to 3.8GHz and the performance was closer to a then-$300 E8400 than the low end Pentium I'd purchased.
The last CPU I overclocked was an i7 2600k, which was my daily driver until last year (!!!).
On one hand, I miss that crazy era, but on the other hand I understand why it had to go away. Intel just don't have the sheer performance advantage to be able to gimp their own chips like that any more.
Edit: Just remembered some of the gimping AMD did, like the HD 6950 that could be upgraded to a 6970 with nothing but a BIOS flash, and the "3-core" Phenoms with a 4th core that could be unlocked in the BIOS.