http://mashable.com/2007/08/10/gmail-toolbox/

GCount – An application for Mac OSX that alerts you to new mail.

gDisk – An app for Mac OSX giving the ability to turn Gmail accounts into storage drives.

gFeeder – A small ticker for your Gmail that will let you know at a glance what’s coming in for you.

GetMail – Allows you to migrate your emails from Hotmail to your Gmail account.

Gmail Drive – Another tool for turning your Gmail account in to a virtual storage drive, this one works directly from inside Windows Explorer.

Gmail Loader – For Windows or Linux, aids you in loading your email to your Gmail account.

GMailFS – Turns your Gmail space into a virtual drive that your Windows Explorer can see.

GotMailG – A Mac widget to display the number of unread messages in your inbox.

POP3/IMAP server for GMAIL in JAVA – Allows you POP3 access via your favorite mail reader to your Gmail account, IMAP still in development.

Send To GMail – With one click, you can back up any file you want from your computer to your Gmail account.

Firefox Tools

   Better Gmail

Better Gmail – Combines some of the most popular Greasemonkey scripts in to one extension. Includes things such as skins, saved searches, keyboard macros, and more.

Delegate to Remember The Milk – Adds a button to Gmail and allows you make to-do lists.

Dragdropupload – Allows you to drag your files to the attachment box.

GCalQuickTab – Allows you to quickly change between Google Mail and Calendar with keyboard shortcuts.

Gmail Checker – Allows you to check your Gmail for new messages.

Gmail Manager – Allows you to not only manage multiple accounts, but know how much mail and spam they have.

Gmail Notifier – Get notifications of new emails right in your browser.

Gmail RTL – Adds direction buttons to your email.

Gmail S/Mime – Allows you to send and receive encrypted emails.

Gmail Skins – Change the look of your inbox, insert HTML tables, use emoticons and more.

Google Toolbar – Official Google release of a toolbar with Gmail features.

Gspace – Turns your Gmail account in to a virtual web drive that you can upload files to and store them.

gTalk Sidebar – Incorporates Google Talk into your sidebar.

GTDInBox – A Getting Things Done tool that integrates into Gmail, helps you get organized.

WebMail Notifier – Checks your Gmail account for unread emails; works with other email services also.

Greasemonkey Scripts

   Gmail Encrypt

Clean Gmail Print – Removes the Gmail logo when you go to print out your email

Folders4Gmail – If you prefer the old style of sorting mail in folders over labels, this is the script for you.

Force HTTPS for GMail, GCal, and GDocs – Forces the corner links in the Google apps to change over to HTTPS.

gDate2Cal 0.1 – Turns dates in Gmail in to links to take you to that day in Google Calendar.

Gmail + Reader Integration – Lets you use Google Reader with your Gmail account.

GMail Attachment Icons – Shows you an icon that lets you know what type of file is attached to an email without opening it.

Gmail attachment reminder v2.4.1 – Reminds you to attach a file your email if it appears you haven’t.

Gmail Contact List – Adds a contact list near the “To” field in compose.

Gmail Conversation Preview – Right click on a conversation to get a preview of it in a bubble.

Gmail Date Search – adds a Search by Date button to Gmail’s search form

Gmail Delete Button – RED Color – Turns the delete button red so there is less chance of hitting it accidentally.

Gmail Emoticons! – Puts an emoticons button next to the “Check Spelling” button to allow you to enter numerous different smilies.

Gmail Encrypt – Allows you to encrypt outgoing emails and create encryption keys.

Gmail FavIcon Alerts – Turns the Gmail favicon in the address bar in to a new mail alert.

Gmail from address selector – Lets you change the reply-to email address depending on who you are sending to. (i.e. sending to someone like “work.com”, you choose your work address as the from address)

Gmail Full Width – Removes the advertising block on the right so that your mails take up the full width.

Gmail Google Calendar sidebar – Adds your Google Calendar to your Gmail display.

Gmail Lite – Removes ads, the footer, stars, and chats.

Gmail Macros – Allows you to create more keyboard shortcuts to speed up your Gmail experience.

Gmail Multi-User Login – For those of you with more than one Gmail account, this will give you a drop down menu with all of your accounts listed.

Gmail Multiple Signatures, plus Float – Will change your signature based on what return address you enter.

Gmail – One Click Conversations – Allows you quick access to the most recent conversations with a contact via a one click button.

GMail Header, Navigation and Ads Switcher – Can choose to hide or show the header, navigation bar, and ads by clicking a button.

Gmail – Zip Attachments and Download – Allows you to download all the attachments of an email by merely clicking on the paper clip icon.

GmailTo -Opens mailto: commands from websites in Gmail compose.

Google Air Skin – Script to use Air Skin in Gmail, Google Calendar, on Google.com, and Google Reader.

Label Colors – Lets you choose the colors of various labels so you can tell them apart easier.

Plaxo4Gmail – Allows you to sync your Gmail with your Plaxo info.

Saved Searches – Allows you to save a Gmail search so that you can just click on the saved search to do it again.

Miscellaneous

   e-mail icon generator

GBooks – Allows you to turn a Gmail account in to a bookmarks server.

gExodus – Allows you to export your emails from another account to your Gmail account in mbox format.

Gmail icon generator – Allows you to create a small graphic of your Gmail address to place on websites so that you may display your address without spam bots reading it.

Gmail Music – Store MP3 files in your Gmail account and play them on any computer.

Gmail Todo – Turn your Gmail into a helpful “To Do” list.

PhpGmailDrive – Retrieves a list of attachments from a Gmail account and enables you to list them on a website or blog for others to download.

You’ve Got Gmail – A plugin for the Trillian messenger program that lets you manage your accounts.

Very useful!
Thanks :)