Gspace - multi-platform firefox addon that turns your gmail account into powerful online storage tool
Gmail Search Extension - search your gmail account directly from the firefox search bar
GcalQuickTab - toggle between gmail and google calendar with keyboard shortcuts
GTDInbox - turns your account into flexible and feature-loaded ToDo-management service (GTD = Getting Things Done)
GMail Manager - manage multiple GMail accounts right from your browser
DragDropUpload - this handy extension lets you quickly attach one or more files to your emails by dragging them over the “Attach a file” link
CustomizeGoogle - removes ads, switches you from http to https, hides the spam counter, quick contacts box, invite box and forces fixed font
GmailSkinz - powerful firefox addon with lots of cool features:
- change the colour of your inbox(from green to dark green),
- view weather, google calendar and news headlines along side your emails,
- integrate any Google IG module to sidebar menu(Awesome),
- insert images from your Picasa web album in to emails,
- insert smileys,
- change the attachment paperclip to an icon indicating the type of attachment,
- make the navigation (Inbox, Starred, Sent Mail, etc) horizontal,
- hide various page elements (invite panel, page footer and your email address from the top right of inbox).
- complete integration with Gmail: addon settings can be changed directly from the Gmail settings page.
Google Toolbar with Gmail search - Official Google toolbar featuring one-click Gmail search
Gmail Spam-count Hide - hides spam count link
GMail Attachment Icons - adds attachment icons to emails, letting you see what kind of attachment an email has (HTML,JPG, PDF….) [ update: ‘Attachment icons’ isn’t a standard greasemonkey script, it’s an addon for a similar Firefox extension known as Stylish. However, installing it on greasemonkey should be fine as well] Google Reader Integration - integrates Google Reader into Gmail. Highly Recommended (demo)
GmailTo - forces all ‘mailto’ links to open in GMail
Conversation Preview - handy addon that lets you quickly preview message contents right from the main inbox window. See demo below
Gmail Macros - adds extra keyboard shortcuts to Gmail (i.e. “t” for move to trash, “r” for mark as read, “p'’ for mark as read and add to archive…etc).
SSLGoogle - force google mail, docs and cal pages and their intra-links to a secure connection(HTTPS)
Desktop APPS and Web TOOLS:
GmailDrive - adds a new virtual drive(’Gmail Drive’) to ‘My Computer ‘ folder and uses gmail as its storage medium. Drag’n drop files Gmail Drive and it will store them on your gmail account as if on additional hard drive.
Gmail ToDo - handy desktop application that stores entered todo’s in a todo label on gmail
GMail for Mobile - access gmail from your mobile
GmailFS - mountable Linux filesystem which uses Gmail as its storage medium
Google Talk - Google’s IM service which is fully integrable with GMail
You’ve Got Gmail - Gmail plugin for Trillian IM client
TIPS and HACKS
Unbl0ck - use this tool to access gmail from places where it’s blocked (work, school…)
Gmail icon generator - creates an image of your email address to minimize spam exposure
Gmail To Thunderbird - how to access your Gmail using Mozilla Thunderbird
Shortcut List - handy, printable list of keyboard shortcuts for the Gmail
Hotmail Contacts Import - explains how to import contacts from Hotmail to Gmail
Gmail trick that explains how to find that ’someone’ who gives away your email address to spammers
Attach any file type - bunch of tricks showing you how to attach and send blocked file types, i.e. ‘*.exe’
JimsTips - huge collection of basic gmail tips
Update: What to do if your Gmail is nearly full? See here.