I recently discovered a simple tool that has lots of power behind it. rrdtool is a round robin database that stores time dependant values and easily graphs them. It is a database where you insert values at consistent intervals and the query results are in the form of a graph. It is a round robin database because it only saves a set number of values and overwrites the oldest one every time.
Now that I pointed out what it is, let us talk about what we can do with that. The first thing that jumps out (and what it was designed for) is performance monitoring. You can set up a task to save the CPU, network, disk, and ram activity to this file every 5 sec and then generate charts to display it. Any thing you can get a counter on. Computer temperature, event log errors, ping times, terminal server connections, and anything else you can think of. Things like the daily temperature, number of visitors to your office, spam messages, or even your daily bank balance.
This is great for performance monitoring because the database automatically discards old data. To put it another way, it only keeps the data for as long as you think its important. Once you define how much you want to store and at what resolution, the database is then set in size. It never grows or shrinks.