Mac only: Apple's Mail app can do a lot of things well (after all, it is one of your favourite email clients), but it annoyingly opens everything in a new window. MailTabs is a free plugin that adds tabs to Apple's Mail.app.
The plugin is quite simple: just download it, install it, and the next time you open up Mail, you'll see a tab bar below the toolbar. Double-clicking on messages will open them in a new tab instead of a new window. It isn't the prettiest plugin around (the tab's colour looks a little awkward since it doesn't match the toolbar), but overall it really makes Mail nicer to use — no longer will you end up with separate windows cluttering up your desktop when you're trying to root through your email. Hit the link to check it out.
MailTabs is a free download for Mac OS X.