world4search is a search engine for the textboards that historically made up world4ch but are now part of 4chan proper.
Several scrapers already existed for Shiichan in general and world4ch specifically (/prog/scrape being the first and most popular), but they tend to dump a board's content into a local SQL-style database, which is difficult to query and less than convenient for the casual discussionist. A search engine has a lower entrance barrier and doesn't require that everyone maintain their own index.
If all you want is to use the search engine, you'll find a running copy here.
If you want to deploy your own search engine, try the wiki.