Back in 2006, the introduction of Intel's Core 2 Duo, based on the Conroe design, allowed the company to reclaim the performance crown, simultaneously ushering in a golden age of overclocking. If we dedicated a page to every model in the line-up with exceptional scalability, this story would be at least twice as long.
Let's start with the budget-oriented Pentium Dual Core, essentially a Core 2 Duo with its L2 cache cut to 1 MB. The Pentium Dual Core E2140 (1.6 GHz) and E2160 (1.8 GHz) were selling for $80 and $90 (respectively) on release, and 3.0+ GHz was an easy target. The Core 2 Duo E6300 (1.866 GHz) was less than $200 when it launched, but could be pushed into the 4 GHz range, taking on the $580 Core 2 Duo E6700 (2.667 GHz) flagship.
Later in the Core 2 product cycle, the Wolfdale refresh included a 45 nm die shrink, allowing CPUs like the 3 GHz Core 2 Duo E8400 to break 4 GHz with little resistance. By no means does this covers all of Intel's Core 2 models, but I don't remember any that weren't good overclockers.