Multi login yahoo,gmail,hotmail - Add a drop-down box with your ID's to the WebMail login form.
Gmail and Google Apps Sidebar Remover - Removes the ads from right side of the screen in the conversation view and moves the thread links from the top of that pane over to the left side, so they are still usable. Works with both the old Gmail and Google Apps, as well as the new Gmail.
Gmail 2.0 Multiple HTML Signatures - Works for Compose Mail, Reply, and Forward.Create a different signature for each email address you use in your gmail account
WebMail Notifier - notifies you of new email on your webmail accounts, such as Gmail, Yahoo, Hotmail and others.
Mail Notifier - email notifier for POP3 accounts.
Contacts Manager - simple manager for your online contacts.
eWebMail Plus! - enables rich text editing for Yahoo! Mail, Hotmail and MyWay in Firefox.
Send Page By Email - easily send the page you’re currently viewing via email.
Email This! - another plugin that lets you mail the currently opened page to someone. Supports GMail, Yahoo, Hotmail, Outlook Express, Thunderbird and other clients.
Amazing Webpage Emailer - yet another web page mailer, with some advanced options; for example, it will correctly send password protected and session-specific pages.
SendStory - send pieces of text from a web page via email.
TrashMail.net - a plugin which creates disposable email addresses and lets you easily paste them into forms.
Temporary Inbox - another plugin that gives you an indefinite amount of disposable email addresses.
Tempomail - yet another temporary email creator.
Spamavert - a highly customizable plugin for creation of temporary email addresses, which should minimize spam in your real inbox.
Send Tab URLs - mails addresses of all currently opened tabs to any recepient.
Netscape Sitemail Notifier - a plugin that notifies you when you receive new mail on your Netscape account.
Gmail Notifier - notifies you of new emails on your Gmail account.
Better Gmail - Lifehacker’s compilation of scripts to improve Gmail functionality.
GTDInbox - if you’re into GTD (getting things done), then you might want to check out this GTD-styled inbox for Gmail.
AJAX Yahoo! Mail - adds AJAX support to the classic Yahoo! mail and lets you preview new messages on your account with a single click.
YMail Notifier - notifies you of new e-mail on your Yahoo! account with a tiny icon in your status bar.
MailNull Now! - easy interface for creating new addresses on Mailnull, an email anonymizer service.
Gmail Skins - a number of extra features for Gmail as well as some skins.