E6600 for my "build-in-progress" [currently testing components in a bare midtower chassis.]
Until I can fully implement my cooling solution, I run it variously at 3.24 Ghz, 3.0 Ghz, and 2.8 Ghz.
The motherboard provides an ability to save two BIOS "profiles" in the BIOS flash memory, and select them from the BIOS setup menu. So right now -- 3.0 Ghz while I see how much more I can tighten the memory latencies.
Also running a 3.4E Prescott at 3.7 Ghz, a P4 3.06 (533), a P4 2.533 (533), a 1.8A P4 (400), and a P3-750.