Mac: Os Track is a free app you leave running all the time in order to chart your historical system resource usage in a visually appealing but not-all-that-detailed way. Why would you do this? You can easily see if one app or another is hogging up your CPU or RAM way more than it should by looking at it historically, rather than just at discrete snapshots in time.
From just a quick 20 minutes worth of use, Os Track seems pretty nifty, and gets even more useful the longer I have it running to collect data. The CPU usage chart seems like it's more-or-less accurate, but for some reason it's giving me only about a 27% memory consumption when Lion's own Activity Monitor says that I've 13.68GB used out of 14GB total. This could very well be because I don't have enough data and Os Track needs to be running longer for it to be accurate. But it is quite nice looking, and that's worth something.
You've got the choice of 7, 15, 30 and 60 day views, and you can either view all your apps, or go in and select select one individually to see how much of your resources it has been using over time.