The following uses for mercury are gathered from a number of sources as well as from anecdotal comments. I'd be delighted to receive corrections as well as additional referenced uses (please use the feedback mechanism to add uses).
Mercury is used in laboratories for making thermometers, barometers, diffusion pumps, and many other instruments. It is used for mercury switches and other electrical apparatus. It is used as an electrode in some types of electrolysis and for making batteries (mercury cells).
Gaseous mercury is used in mercury-vapour lamps and advertising signs. Mercury is important in some types of industry, for instance for caustic soda and chlorine production. It is used for making some pesticides and antifouling paint. Mercury is also the basis of dental amalgams and preparations.