After migrating my company's email away from Gmail a few years ago, I've become firmly entrenched with Apple's Mail.app on the desktop. Everything works great, but I do miss having access to Rapportive's Gmail extension. It's great at providing extra information related to the person you're emailing with. To make up for this lack of functionality, I've created a quick AppleScript that automatically opens up a new Mail window and finds any previous conversations I've had with the sender. I use it all the time whenever a customer emails so I can quickly see any past conversations.
I've built the script into an Alfred workflow that lets me run it via a keyboard shortcut. For Alfred users, here's the workflow. And the raw AppleScript is below for those of you using FastScripts or some other script launcher.