If you've ever had suspicions that someone may be reading your email but you've never seen proof, weblog MakeUseOf details how to set up an "electronic tripwire" in your email account to keep you aware of email break-ins. The trick uses a web utility called OneStatFree to track file requests, and depends on the email snoop reading this email. Since OneStatFree will let you know if anyone tries downloading the file (and will log the time, IP address, and approximate location of the snoop), you'll know the snoop was there even if the he tries covering his tracks by marking the message as unread.
Are you Sure your Email isn't being Hacked? [MakeUseOf]