Thunderbird (version 3.0.x and 3.1.x)

Open external websites in tabs. These extensions are based on the google calendar tabs extension.

google calendar (v. 2.1: updated with new Google Calendar favicon)
google mail
google docs
google contacts
google reader
google wave
google voice

live mail (hotmail)
yahoo mail

Experimental (it might not work properly, due to the very limited Javascript capabilities of Thunderbird 3.x)
Remember the milk

Source: You can easily modify and repackage any of the extensions yourself. The xpi format is basically the same as zip. So you can open the package with any zip utility (7zip is recommended) and start customizing any of the included files.