External-links-in-a-new-window is a project mainly written in PHP, it's free.
de.okanesen.externallinks
With this plugin all external links will open in a new window - XHTML-Valid. All links, in the WCF, which refers on an external page, will opened in a new window by a klick.
Every user can set for himself (Edit profile -> Settings -> Display settings -> External links -> External links in a new window), whether external links should be automatically opened in a new window or not.
This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with this program. If not, see http://www.gnu.org/licenses/.