Excel spreadsheet. I especially like to keep track of the age of my components and consumables. My RD cables, for example, tend to fail at about 3,200 miles because I have 5600 shifters and I shift a lot. I like to replace that cable before it fails out on the road. And, because of ongoing medical treatments, I like to keep track of my mileage during various treatment regimes. I always like to tell my doctors, "I rode 500 miles since our last appointment." That always saves time, since many questions about how I'm feeling and my general health are answered with that short report. Plus, it impresses the heck out of most medical professionals, many of whom are out of shape.