How to use tabs instead of new windows in Safari

By default, any time a web site uses the target="_blank" attribute on a link (like this one), Safari 4 will open that link in a new window. If you’d prefer it to open in a new Tab, follow the instructions outlined in this brief tutorial.
Start by opening a Terminal window (Applications -> Utilities -> Terminal). Enter the following command:

defaults write com.apple.Safari TargetedClicksCreateTabs -bool true

Now restart Safari. Links that used to open in a new Window (like this one) will now open in a new Tab instead.

If you decide you want to change back, just re-enter the command but replace true at the end of the command with false.

Info found at: switchingtomac.com