search.wikimedia.org holds a web service for running title prefix searches for Apple's Dictionary application in Mac OS X 10.5. It is a simple ~100 line script hosted on the main application cluster, which proxies requests to the opensearch API and reformats the results.
There is a contract with Apple requiring this service to be maintained.
The source is in the operations/mediawiki-config repository at docroot/search.wikimedia.org/index.php
Apple requested that we implement this service in October 2007. Mike Godwin negotiated the contract, and Brion Vibber (then CTO) wrote the code and set up the service. It used the TitleKey extension for the first two months, and then Brion migrated it to use the new OpenSearch endpoint. Originally the service ran on separate hardware: first yongle and then ekrem. When it came time to decommission ekrem in 2013, there was some talk of discontinuing the service (phab:T81982). Log analysis showed that it was still actively used. So Tim Starling moved it to the main application cluster, and updated the source, with a view to allowing another 5-10 years of low-maintenance service.