Okay, so I'm confused about the original question....
1) Just to make sure, you're not asking if Safari has tabs at all, are you? You laugh, but people have actually started numerous threads asking why Apple makes a crappy browser that doesn't even have tabbed browsing.

Obviously, it does, and you can configure the basics of how they behave from the Tabs pane of preferences.
2) If you want windows that normally open as new windows to open in tabs instead,
Safaristand does this. It's free, there are versions for both Safari 2 and Safari 3, and it works very nicely. In the first preference pane, on the lower right side, there is a checkbox that says "open _blank..." see the picture:
SAFT also does this.