Another application which may be helpful (depending on your needs) is
Instruments.app included in Xcode.
It doesn't allow a continuous easy monitoring like
iStat Menus or
Activity Monitor do, due to the fact that some user interaction is needed to set up a sample and start recording it. On the other hand you may monitor single applications – a feature the aforementioned apps don't accomplish.
From a programmer's perspective this may be very useful.
Usage: Instruments Help cmd? -> Viewing Collected Data -> Monitoring Multiple Core Usage