Jnana Kumara of Nyak
Tibetan Input Methods translate sequences of characters typed on standard keyboards into Tibetan glyphs. An input method is one of three components needed for a complete Tibetan Unicode Installation.
A very good description of basically all available Tibetan Inputs methods is given at the THDL project of the University of Virginia.
Windows 2000 and Windows XP
Tavultesoft offers a commercial Tool, Keyman, which supports a number of different input methods for Tibetan. Starting with version 7, Keyman is no longer available for free.
A free input method, TISE, is available here.
Vista supports a tibetan input method as part of the operating system. No further software needs to be installed. A Tibetan keyboard can be enabled using "Control Panel / Regional and Language Options".
The UIM package contains a Wylie input method for Linux.
For Mac, MacUIM is a port of UIM for Mac OS-X.