How would you like to see the source code?

FishEye for online browsing of the commit history
Online cross-reference if you want to browse the current codebase, with IDE-like navigation capability.
Subversion repository for checking out the source code. Username 'guest' and password is empty. Also see Building Hudson or Checking out existing plugins.
Git repository is also available if you'd like to contribute patches that way.
Source bundles for released versions of Hudson contains the exact source code used for producing release versions of Hudson.
Javadoc is of course available too.

Source code