This is an early release, the extension is primative and does not work well with all websites. Cavaet emptor.
If you don't know how to install this, make sure you are running Safari 5.0.1 or higher, then download and expand the archive. You will find two extensions, double-click on each one individually to install it into Safari.
Known Bugs & Future Enhancements
I will make it an option to open in a new tab by default, if that is your preferred behaviour. For now, right-click and choose "Open in New Tab".
There is no reliable way to shift over the contents of the existing web page, although I do try. You may find some sites have their right-hand sides covered up.
One thing that has always pissed me off about Safari is its disrespect for metadata. Prominent among those metadata are the navigational links often provided by web authors for traversing a sequence of documents.
With the advent of Safari 5 and its modest Extensions API, I have finally been able to implement something that I could not submit as a WebKit patch: A navigation bar for metadata links.