Mozilla Thunderbird: Thunderbird 0.4 is out. The attachment icon (the paper clip) has been broken out to its own column (instead of being overlad on the envelope — you could never see it) so you can sort on it now, and the envelope now includes “forwarded” and “replied” arrows like Outlook.
That’s what grabbed me on first glance, but the complete list of improvements is impressive. Aong them: