Huh, I just tried it and it seems to do what you want, although I will add that it feels like the Safari 8.0 support is a bit half-baked right now...the side/vertical tabs look kinda ugly and it doesn't suppress Safari from displaying the normal horizontal tabs across the top, so you actually will have every tab showing up on the UI twice.i cant quite figure out what it is from their website....
It's easy to remove it if you don't like it, so why not give it a shot?
1) Download EasySIMBL from http://github.com/norio-nomura/EasySIMBL/releases/download/EasySIMBL-1.6/EasySIMBL-1.6.zip
2) Download SafariStand for Yosemite from http://hetima.com/safari/stand/SafariStand8.0.205.zip
3) Quit Safari.
4) Unpack EasySIMBL-1.6.zip and move EasySIMBL.app to Applications.
5) Unpack SafariStand8.0.205.zip
6) Run EasySIMBL.
7) Check the "Use SIMBL" checkbox at the top.
8) Click the "Show Plugin Folder" button in the bottom right corner.
9) Drag the SafariStand.bundle that was unpacked from the SafariStand ZIP file into the plugin folder.
Now close EasySIMBL, launch Safari, and you should see a new Stand menu show up while Safari is running. Under the Stand menu, click Sidebar, and voila, as they say.
To kill SafariStand, just open up EasySIMBL and uncheck the SafariStand plugin. Optionally, open up the plugin folder and move SafariStand.bundle to trash, uncheck "Use SIMBL", close EasySIMBL, and then move EasySIMBL to trash, too.
-- Nathan