AIM9
The AIM Independent Resource Benchmark creates exercises and times for each component of your UNIX system. Each of them is run independently, for a total of 58 subtests. Then it will provide you with absolute processing rates. The results are expressed in operations per second, I/O transfers, function calls, and UNIX system calls.
top
This application gives you a real time look at your system. Good for identifying the dreaded and annoying system bottlenecks, scourge of many a system admin.
iostat
This software gives you the basics on your disk utility and your CPU loads. The output can be a bit cryptic, but it is worth trying.
tcpdump
If you suspect that a performance issue it related someone who is accessing the network without permission then this is the tool for you. Sure, it sounds like it is only for servers, but it can be of use to a skilled home user as well.