Regmon is a registry monitoring utility that will show you which applications are accessing your registry, which keys they are accessing, and the registry data that they are reading and writing - all in real-time. This advanced utility takes you one step beyond what static registry tools can do, to let you see and understand exactly how programs use the registry.

Note that Vista and above are encouraged to use Process Monitor.

Note: Main site link offline (thanks WolfGhost). Linking to Softpedia.