AMD's Athlon chips are particularly popular for overclocking. Intel's early Celerons were overclocking marvels. Unfortunately, those early Celerons are hard to find, but folks have overclocked 566-MHz Celeron chips up to 900 MHz. These days a mid level Pentium 4 is a very popular choice; [H]ardOCP has some great Pentium 4 1.6 GHz mobo combos that can easily hit 2 to 2.4 GHz

You can even tweak Pentium III Coppermines. The faster your chip, the less likely it will respond to overclocking, however.

Check Overclockers.com's CPU Database for info on how fast folks have overclocked a given chip.