QuoteFix is a plug-in for Apple Mail which fixes some issues with replying to e-mail:
• it tries to remove the signature from the original message QuoteFix will only remove signatures if they start with the "internet standard" signature separator "-- " (that's hyphen hyphen space, followed by an end-of-line).
• it removes certain unnecessary empty lines
• it positions the cursor below the original message, instead of above it (in other words, bottom-posting instead of top-posting)
• it can (optionally) prune nested quotes from a specific level and above

Requirements:
OS X 10.8+
Intel