The Saft free plugin can supposedly do that (last download is up to Snow Leopard only).
From the article safari sidebar :
goes into the Library>InputMangers folder replacing the previous
version. Once launched, Safari then has the ‘Show Sidebar’ command in
the Safari menu (this is a temporary position).
According to your remark that Saft doesn't work on 10.7, here is an extract from the article
Make SAFT work with newer versions of Safari Web Browsers :
First, quit out Safari and then open the top-level Library ->
InputManagers -> Saft folder. Control-click on Saft.bundle and choose
Show Package Contents from the pop-up menu. Within the Contents
folder, open info.plist, either in Property List Editor or your
favorite text editor. There is an option for SaftSupportedSafariBuild
within which you can re-enter the version number from Safari's About
Box. Change that, save, and relaunch Safari to enjoy Saft goodness.
[robg adds: Depending on how much Safari has changed, you may find
that Saft still won't work, or won't work properly. The Saft author is
usually quite good about keeping up with Safari updates, so this hint
should only be needed if that ever changes.]