#translatewiki.net, they largely scratch their own itch. Some messages are more visible then other; they are for instance the "most used messages" for MediaWiki. Other messages are special for other reasons.
When the Babel extension messages are localised it is special. These messages are already among the most localised messages and they provide a nice indicator that shows if you can actually read a language properly.
It is as wonderful to notice when the messages for WebFonts and Narayam are being localised in the Khmer language.. They are tested and they are made ready for production.