Tibetan Input Method
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.
Overview
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 free Tool, Keyman, which supports a number of different input methods for Tibetan.
Windows Vista
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".
Additionally, Tavultesoft is currently developing a new version of Keyman which supports a number of different input methods for Tibetan including Wylie.
Linux
The UIM package contains a Wylie input method for Linux.
Mac OS-X
For Mac, MacUIM is a port of UIM for Mac OS-X.