How do I import data into Craft? What's the best known way to import masses of data into Craft?

Are there tools that support the migration of content/users from say XML, JSON etc.
There is craftimport which is an 'unsupported' starting point for importing entries from an XML source.

https://github.com/clearbold/craftimport

There is also Import:

https://github.com/boboldehampsink/import

which can import from CSV sources.

Check out http://straightupcraft.com/craft-plugins for a list of current Craft plugins.