One of the things that annoy me on Safari is the impossibility to change Safari's behavior for opening links in new windows.
I got used to using tabs for everything on a web browser. So, Safari's behavior of opening "_blank" links in a new window was difficult to accept. After searching the web browser's preferences and being unable to find such an option I decided to look for a solution on Google.
I ended up finding this neat plugin that, besides showing a sidebar with thumbnails of the currently opened tabs (as the picture on the left shows), allows tweaking some of the options of Safari, including the behavior to open links in new windows.
To install this plugin, just download the ZIP file, unzip it, and (with Safari closed) copy the SafariStand folder into ~/Library/InputManagers. Then open Safari and you'll notice a new Menu Item next to the "Help".
On this new menu choose "SafariStand Setting..." and in the General tab choose the option "Open '_blank' Link in New Tab". This will avoid new window links to actually open on a new window :-)
Source: The Mac Newbie