GDocBackup is simple utility to backup (export) all your documents from Google Documents 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 uses the official Google Data APIs (.NET version). The GDocBackup.x.x.x.BIN.zip file contains all required files.
Link