Windows only: Outlook Add-in Sender's Time Zone makes easy work of dealing with recipients in multiple time zones—no more calculating times in your head.
Once installed, the add-on puts a small display on each message window with the time zone of the sender, the number of hours behind, as well as the time since the message was sent—it's one of those add-ons that's so useful you aren't sure why they didn't include in the first place.
Sender's Time Zone is a free download for Windows, requires Microsoft Outlook. Gmail users can get the same functionality by taking advantage of the Gmail Time Zone Notifier Labs option. Thanks, Scott!