Most projects seem to regard those update locations as a kind of secret. At least most of the projects I use regularly (see below for a list of them). For some update locations you can search ages. And if you need the unstable update locations even longer.
Some Hint for eclipse plugin developers: If you have an update location for your eclipse feature/plugin, please post it on your plugin homepage in a prominent place. That would help a lot. Also please post your unstable update link, especially if your stable version is mucho out of touch with reality.
Anyway, for some of the update locations below I have searched for ages. Because I want to avoid that in the future here is a list: