GDocBackup stopped working on April 20, 2015. Google stopped the old APIs used by GDocBackup.
It's no longer actively developed nor maintained. So there will be no support, fixes or updates. Sorry. If interested in source code, it will remain available at https://code.google.com/p/gdocbackup/

Fell free to contact me if you need more information.

UPDATE
If you use Google Apps and you're looking for a solution like GDocBackup, take a look at DriveExport. It's very similar to GDocBackup. It only works with Google Apps™ accounts and not with personal GMail/Drive™ accounts.

http://www.driveexport.com/

**** FROM THE OLD WEB SITE ****

Google Docs™ backup utility

Overview

GDocBackup is simple utility to backup (export) all your documents from Google Docs™ to local disk. It's open source, freeware and licensed under the Apache License Version 2.0 It does a simple backup: for each document in Google Docs, it downloads the document if the document is not present on the local disk or if it has a different date.

GDocBackup runs on Windows (tested on XP, Vista and 7), Linux (tested on Ubuntu 8.10 and OpenSuse 11.1) and Mac (tested on OS X 10.5.6). Required .NET or Mono.

IMPORTANT: GDocBackup uses the official Google Data APIs (.NET version). So the quality, accuracy and correctness of the exported files is out of the control of GDocBackup. If you encounter problems in the exported documents (i.e. the exported doc is not similar to the original on Google Docs), contact Google Docs support forum.