QuoteFix for Mac (hosted at Google Code) is a plug-in for (Snow) Leopard's Mail.app, and fixes some issues when replying to e-mail:

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

The plug-in uses the private plug-in API for Mail.app, for which a very useful description can be found here. The plug-in is written in Python and uses the Objective-C/Python bridge which is a standard part of Mac OS X. 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

Next, download the plug-in and unpack it in ~/Library/Mail/Bundles/ Lastly, quit Mail.app if it's running, and start it up again. The plug-in has no user-serviceable parts inside (read: there's no configuration). In case you run into any problems, just remove QuoteFix.mailbundle from the bundle-folder above and restart Mail.app.