Not suprisingly, we recommend our own publisher - the Tibetan and Himalayan Digital Library (www.thdl.org) in general as a place to begin. It has excellent resources on Tibetan places, but also other resources on Tibetan Buddhism, medicine, music and more.
For general knowledge on Buddhism
For superb scholarly resources on Tibetan texts, lamas and places, we recommend the Tibet Buddhist Resource Center (www.tbrc.org).
For specific schools or monasteries of Tibetan Buddhism:
David Germano, Elizabeth Tseng