In OS X El Capitan, due out this fall and now in public beta, you can now ‘pin’ tabs in Safari. They get a dedicated space at the left of the tab bar, they always remain active, and they stay persistent between Safari sessions. If you quit Safari or even restart your Mac, they’ll be there the next time you open Safari.
To pin a tab, simply drag it to the far left of the tab bar. Eventually it will shrink in size and only show its favicon, which means it is now pinned.