A very AJAX-enabled free online e-mail service.
Still in beta, and offering over 2.6gb of mail storage.
Includes targeted, relevant ads, most of which are blockable if you use adblock and MozillaFirefox.
Also relevant is the Gmail Notifier for MozillaFirefox, found at http://www.nexgenmedia.net/extensions/
At the moment Gmail access is only available with an invite, but these are easily found.