Statistics Collection Daemon for filling RRD Fileshttp://verplant.org/collectd/
collectd is a small daemon written in C for performance. It reads various system statistics and updates RRD files, creating them if neccessary.
Since the daemon doesn't need to startup every time it wants to update the files it's very fast and easy on the system. Also, the statistics are very fine grained since the files are updated every 10 seconds.