The beer bug is a great tool that measures the temp of your beer and relays that info to the web. It has a great iphone app that allows you to monitor your progress as well. Temperature is important because yeast puts off heat when it works, and that heat can be used as a proxy to tell when the yeast is done working.

The beer bug is good for a person who likes to get into the nitty gritty of data, and tries to understand what input causes what output.