Your best bet is sar. It's available for Linux, Solaris, Irix, HP-UX, AIX, the dreaded SCO, and probably more *nix. Sar comes with a ton of options for customizing output, so it's my main tool for measuring system performance. I wrote my own wrapper for parsing out only what I needed using a simple split (sorry, work won't let me share), then I printed everything to a file as comma-separated values, making it easy to process the data into reports, graphs, what have you. There's also a good article (code included) here about using Gnuplot to generate graphs from sar output.