LaTeX is a typesetting tool widely used in the scientific world. It runs on all platforms (including Windows, MacOS, Unix, Linux)
LaTeX is usually available already in a standard Linux distribution. It is able to deal with a large number of languages and scripts, including Tibetan. In order to produce Tibetan text, one needs the package ctib (http://www.ctan.org/tex-archive/language/tibetan/ctib/). Everything that is written after the directive tib is rendered with the Tibetan font provided by the package. Input is a slightly modified Wylie transliteration. This is a very easy way to produce Tibetan text from material given in Wylie. Note that LaTeX is NOT a WYSIWYG utility. All rendering is done in postprocessing steps. For rendering existing Wylie texts this is actually an advantage, as you don't need to retype to get the correct stacks etc.