Evander Holyfield, he was never the big 1 punch KO artist his peers were but he could put some pop in his punches. (Though he was usually aided by his head)

Bernard Hopkins, same as Evander.

Sergio Martinez, though the Williams KO was a big time power punch.

Felix Sturm, he kept De la Hoya at bay for an entire fight. Marked him up pretty bad too.