A spellchecker for Tibetan and Libre- or OpenOffice
A spell checker for Tibetan can be added to Libre Office, Open Office or any application using the hunspell library (like InDesign, most text editors, Firefox, etc.). More information on the website.
The syllables present in the spell checker correspond to the Classical Tibetan traditional grammar, plus some very common names (ཀརྨ, etc.). See this repository for documentation elements.
OpenOffice and LibreOffice
For those who don't want to build the extension themselves, here's a prebuilt version:
Download the .oxt file from the extension page, then start LibreOffice.
Use Tools / Extension Manager ... / Add... to install the extension
Restart LibreOffice to activate the extension.
Type some Tibetan, and make sure that the language is set to Tibetan (PR China).
You can set the language using the menu Tools / Language / For all text.
As a dictionary used by all application
hunspell-bo package (under Debian and derivates).
Follow the instructions on the website.
A spell checker with similar features also exists for Dzongkha: