Benchmarking performance is something that every admin will need to do on a fairly regular basis.  My list of "go to" open source tools are as follows:

lmdd - lmbench : An excellent tool for raw disk speed metrics.
Good information about lmbench can be found here.

iozone : A very good tool with built-in charting.
Good information about iozone can be found here.

iperf : TCP/IP bandwidth benchmarking tool that is very configurable.

A listing of other Open Source tools can be found on this site.