An often overlooked utility that's built right into OS X is the Activity Monitor.app.
It lets you view all running processes as well as their memory and cpu usage allowing you to narrow down which apps are misbehaving.
Another handy feature is the updating dock icon.
By control-clicking or right clicking on the Activity Monitor dock icon you can set it to display CPU usage, CPU usage history, network usage, disk activity or memory usage in real time...
Activity Monitor.app lives in Applications -> Utilities.