Gadolinium is a heavy lanthanoid used in many storage devices. It has a high neutron absorption which makes it useful shielding for nuclear reactors. Gadolinium also is used as a contrast agent in magnetic resonance imaging. It is produced from the minerals monazite and bastnasite.
Molycorp - http://www.molycorp.com/ - Molycorp is a mining and marketing subsidiary of Chevron (NYSE: CVX) that produces gadolinium, in addition to molybdenum and other rare earth elements.
Avalon Rare Metals (TSE: AVL)
Metall Rare Earth Limited - http://www.metall.com.cn/gd.htm Commerce Resources Corp. (CVE: CCE)
Great Western Minerals Group (CVE: GWG)