The SVN repositiory lives at: http://svn.geograph.org.uk/svn/, below are links to the most important part of the repositiory:

http://svn.geograph.org.uk/svn/trunk/: The main website codebase - checkout this if you wish to start your own project. This will usually contain the most stable code. Please contact us if you want your own branch
http://svn.geograph.org.uk/svn/branches/british-isles/: The Geograph British Isles branch. The developers might have developmental stuff, that only once stable is merged back into the trunk.
http://svn.geograph.org.uk/svn/branches/ger/: The Geograph Deutschland (Germany) branch.
http://svn.geograph.org.uk/svn/apps/trunk/JUploader/: Java based bulk uploader - read more
http://svn.geograph.org.uk/svn/apps/trunk/tracker/: Customisation of Rivet Tracker - which we use to distribute torrents of images. Rather than setup your own tracker you are welcome to use ours for geograph related downloads - please contact us.
http://svn.geograph.org.uk/svn/modules/trunk/perl/: Perl based module for interacting with the Geograph API - also found on CPAN
http://svn.geograph.org.uk/svn/modules/trunk/picasa/: Picasa button - for bulk uploading to Geograph.