Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Imagus - The extension is intended to complement image features that (I think) is missing from Opera by...
Roundcube - Web-based IMAP email client
Image Max URL - Help find larger or original versions of images, including a customizable image popup feature
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Thumbnail Zoom Plus - The official site of the Thumbnail Zoom Plus add-on for Firefox