Where to download api documentation ?

Nuno,

1/
   > Is there a .chm version ?

http://delynnberry.com/pages/rails-chm-documentation/

(MacOS X users can read chm files with 'chamonix'
   http://sourceforge.net/projects/chamonix/
)

2/
There's also DevBoi, a Firefox plugin that gives easy access to
various documentations.
   http://devboi.mozdev.org/
You can even have it install local/offline version of some docs.

Alain