BGS Best1's data collector has now apparently been combined with BMC Patrol so that (despite my personal dislike for Patrol) it might be a goer. TBYB.
Alternatively simply using the various *stat's with some noddy bits of perl or shell-script will give useful date/time-stamped tables which you can then graph using spreadsheet software or gnuplot, and are quite amenable to importing into databases or packages like SAS.
RRDTool is also a likely candidate for the kind of performance-tracking trending and capacity planning that you might want to do.
RRDTool doesn't collect data though, just helps you work with time-based data.