Relevant for wikis on MediaWiki 1.5 only:
I have coded and made operational an extension that can be used to add a search box, or a "create an article" box, to any page:
http://meta.wikimedia.org/wiki/Inputbox_extension
The search functionality is primarily of interest to Wikipedia, where those Wikipedias that would like to now have the option to put a prominent, Google-like search box on their Main Pages.
The create an article functionality is currently trialed on the English Wikinews to make it easier for newcomers to start writing stories. It supports preloading the newly created page with the source from another page, and also adding a custom tag at the top. These two features are independently available:
http://en.wikipedia.org/wiki/index.php?title=FJADOVAFD&preload=Stub&...
loads the text of [[stub]] into the new page with the bogus title.
http://en.wikipedia.org/wiki/index.php?title=FJADOVAFD&editintro=Stub&am...
gives you a modified introduction, which is useful when creating custom edit pages. The two parameters only work when the page does not exist yet.
Best,
Erik