In the latest version of Safari, Apple have added a new hidden preference. Using a Terminal command, you can now enable one-window browsing.

If you use Safari regularly, you may notice that certain links refuse to open in the same window. If a link is set to open in a new window, normally holding down Command while clicking it will make it open in a tab instead. However, in certain places (like Gmail messages) links open in a new window no matter what you do.

This command changes the behaviour such that links that would ordinarily open in a new window now open in a new tab instead. Start by opening up Terminal (located in Applications/Utilities), and paste in the following line:

defaults write com.apple.Safari TargetedClicksCreateTabs -bool true

Press return, and quit and reopen Safari. Now all links targeted for a new window will open in tabs instead. To change back to the normal settings, use the same command but with false at the end instead:

defaults write com.apple.Safari TargetedClicksCreateTabs -bool false

Don't forget there are loads of other tricks for Safari 3, such as enabling the web inspector and bookmarking all open tabs.