AutoIndex is a PHP script that makes a table that lists the files in a directory, and lets users access the files and subdirectories. It includes searching, icons for each file type, an admin panel, uploads, access logging, file descriptions, and more.
Lurker is a mailing list archiver designed for capacity, speed, simplicity, and configurability in that order. Noteworthy features include: google-style searching on all fields, chronology preserving threads, multilingual, and attachment support.
HouseSpider is a Java applet that adds search capability to your web site. It can search by two methods, by spidering through your site or by searching a cached index file. It has 100% i18n (internationalization) support.