QuoteFix can be downloaded from here.
Following the description from the developer's site: QuoteFix is a plug-in for Mail.app which fixes some issues with replying to email.
- it removes the signature from the original message
- 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
Installing the plug-in:
Before installing the plug-in, you'll need to make sure that Mail.app's plug-in support is turned on. For this, execute the following two commands in Terminal.app:
defaults write com.apple.mail EnableBundles -bool true defaults write com.apple.mail BundleCompatibilityVersion 3
Lastly, quit Mail.app if it's running, and start it up again.
In case you run into any problems, just remove QuoteFix.mailbundle from the Bundles folder above and restart Mail.app.
[crarko adds: I tested this, and it more or less works as described. It is very sensitive to the version of Mail that you have.]

