Google Docs & Spreadsheets is one of the best (if not the best) online office suite available at the moment. Google Docs has many interesting and useful features for the end user. Here some of the tools that makes you to get most out of the Google Docs.
OpenOffice.org2GoogleDocs: This extension will allow you to sync OpenOffice documents to Google Docs and Google Docs documents to OpenOffice.
gDocsBar: gDocsbar allows you to manage your Goodle Docs in your firefox sidebar. (Review)
Google Docs Uploader: This is a free application from Google to upload a lot of documents to Google Docs from your desktop. Download the application from here, requires .Net framework2.0
Google Docs Download: This script is available as a Greasemonkey script or as a Firefox Extnesion, it will help you to download all your Google Docs at once to your computer.
Google Web Desktop: It shows your Google Docs as icons in a desktop, you need to grant access to use your Google Docs. I am not sure whether this is of any use but if you love Web OS software you want to check this out.
iGoogleBar is a Firefox extension that adds Google Apps favicons to Google Apps bar, using them as triggers for the Apps respective iGoogle Gadgets.
Do you know any tools or plugins that are not listed here, i would like to know, please share it in the comments.