top is good. I like GKrellM for a nice little rolling graph of several stats (CPU being one of them). Of course there are the simple little gadgets like xload, xcpustate. Then there are non-graphical apps like top, procinfo, iostat, vmstat, etc that have lots of options. Then there are logging apps like sar which is an excellent logger capable of keeping detailed logs and creating reports on just about everything you can think of relating to performance. Of course this is just the short list. These are utilities included with most UNIX/Linux systems (not counting GKrellM).