RRDtool
Suppose you had a process that generated data every second, but only data on the timescale of weeks or months is useful. Some of my friends recently used RRDtool for this. This tool is extremely useful for any kind of time-series and is available under a liberal license (GPL.)
Quoting from the tutorial,
RRDtool refers to Round Robin Database tool. Round robin is a technique that works with a fixed amount of data, and a pointer to the current element. Think of a circle with some dots plotted on the edge — these dots are the places where data can be stored. Draw an arrow from the center of the circle to one of the dots — this is the pointer. When the current data is read or written, the pointer moves to the next element. As we are on a circle there is neither a beginning nor an end, you can go on and on and on. After a while, all the available places will be used and the process automatically reuses old locations. This way, the dataset will not grow in size and therefore requires no maintenance. RRDtool works with with Round Robin Databases (RRDs). It stores and retrieves data from them.