I have heard of some of these CPUs over the years.
An interesting one for me was the "Intel Xeon LV 1.6 D1 (Prestonia)":
The Prestonia core was basically the Pentium 4 Northwood with SMP (symmetric multiprocessing) and HyperThreading added as standard features. With the sub-$200 1.6GHz Xeon drawing a frugal 1.274v, overclockers generally couldn't take advantage of voltage headroom as most boards were voltage-locked. However, simply raising the FSB would net 2.6GHz.
For the more adventurous, three hard mods could yield a 100% overclock (or more!): the U-Wire mod which involved bridging two (1.5v) or three (1.6v) sets of socket pins, the BSEL mod to isolate or break CPU pins and raise the FSB limit to 200MHz, and the vDIMM mod to raise RAM voltage.
Those willing to push the limits of the technology could be rewarded with a 3.2GHz dual processor performance king for around $700 (CPUs, coolers, board, and RAM).
Even without the hard mods, this is a solid CPU with basic FSB raise considering the price.