Below is a screenshot of it in action. You can also test at http://xenmods.com, there aren't many threads created so I recommend using phrases such as "Opening" "Grand" "Hash" "Tag" "Similar" "Thread". It's not case sensitive.
The phrase "Have You Seen?" can be edited in the admin panel under phrases, search "xm_haveyouseen". The matched threads display the title, the author, the time they were posted, the amount of replies, the amount of views, and if there are responses it'll show the time and author of the last post.
If there are no matches, no list is generated. Feel free to try it on this board.
Future plans include making an option to designate if it only searches for threads in the same node, and an option to display similar threads when viewing a thread.
Widget for viewing a thread is done. This is now for sale here : http://xenforo.com/community/resources/xenmods-similar-threads.2219/