Cesium atoms are used in extremely accurate atomic clocks. Barium absorbs X-rays. It is used in a liquid swallowed by patients before getting a stomach X-ray. Lanthanum and other lanthanides are used in carbon-arc lamps in motion pictures. Praseodymium and Neodymium are used in welder's goggles.
Neodymium is used in astronomical spectrometers.
Promethium is used as a power source in spacecraft. Samarium is used in carbon-arc lamps. Europium is used in red phosphors in televisions. Gadolinium is the best neutron absorber and is used in nuclear reactors. Terbium is used in green lasers.