It's easy to make a Firefox, IE7 or Google Toolbar search plugin for your wiki.
Adding your wiki to the search box list in IE7 is extremely easy. Simply click the drop down menu from the search box:
And then follow the instructions on the right side of the page:
http://yourwiki.com/Special:Search?search=Be sure to replace yourwiki.com with your wiki's address!
The back of the search term is
&type=fulltext&go=Find
Power users will notice that you can change these, as they are simply options. Dig into Lucene for more info.
Also, you can make your search plugin auto-discoverable by adding the following at about line 112 of your /skins/ace/Ace.php:
<link rel="search" type="application/opensearchdescription+xml" title="searchTitle" href="pluginURL">
Extend your search and have a search engine of your documents as well.