Manual:$wgEnableScaryTranscluding

Details
Enable scary transclusion (transcluding pages or templates from another wiki). Reasons for its somewhat strange name: It's scarily inefficient. See User:Peter17/Reasonably efficient interwiki transclusion for a better, but currently (September 2010) not yet stable approach.

This setting applies only to pages in the template: namespace; both $wgEnableScaryTranscluding and the iw_trans bit in the interwiki table must be set for this to be enabled.

How to use scary transclusion
You should now be able to type or  to transclude the page from a foreign wiki. You can also do to do a raw transclude, where all templates on the foreign page are expanded as if they come from your wiki instead of the other wiki, or  or  to save the fully expanded code to your wiki. and  will show redlinks for any pages or templates that do not exist locally, however, the basic form will show normal blue links and templates as if they existed locally, though normal wikilinks will connect to locations on the remote wiki.
 * Enable $wgEnableScaryTranscluding
 * Make sure the relevent interwiki prefixes have iw_trans set to 1 in the database.

This will work for non-mediawiki pages to some degree, but the mediawiki software will attempt to parse the non-mediawiki wikitext (such as Moin Moin), or those portions of any html that is allowed by the mediawiki, which will result in varying degrees of meaninglessness.