When you’re using Mail to read email your window looks something like this (but, hopefully, not as blurry):
Direct your attention to the area within the red rectangle in Picture 2:
See those little arrows next to some of the messages? They mean something. The curvy one means you replied to that message. The straight one means you forwarded the message. Ah, but that’s not the tip. The tip is, if you click on a curvy arrow, it actually opens your reply! If you click on a straight arrow it opens your forwarded message. This can save you all kinds of time– rather than searching through your Sent mail to find the reply you sent, you simply click the curvy arrow and voila, there it is.
Yes, it’s been there all along, and yes, I’m sorry I didn’t mention it earlier. I meant to.
Entourage users: you can click the link that says “Show Reply.” Look for it.