The websiteFrame PHP extension
The sourcecode for the extension can be found here.
To add the extension: copy the souce code in a file named websiteFrame.php, and put it in directory /etc/mediawiki-extensions/extensions-available; then execute the following commands:
cd /etc/mediawiki-extensions/extensions-enabled ln -s ../extensions-available/websiteFrame.php
Now you can use an iFrame by putting in code like this:
<websiteFrame> website=http://www.google.com/talk/service/badge/Show?tk=z01q6a-longcode-8k5lm1&w=200&h=60 border=0 height=60 width=200 name=GoogleChatwithSaruman! allowtransparency=true </websiteFrame>
Note that the "=" sign is replaced with = and the & sign by & because the PHP script doesn't like them literally.