Quite often, customers and partners ask about iFilters. For those who don’t know what an iFilter is, well, when a Indexing Service needs to index the contents of a file, it needs to know what the format of the file is. A PDF file has a different format to a DOC or DOCX file. An iFilter file contains the meta-data so that the Indexing Service understand the file. More information on iFilters is available on the MSDN site.

For some good lists of iFilters, visit one of the 2 sites:

http://blogs.msdn.com/joelo/archive/2008/02/07/ifilter-pack-for-indexing-for-sharepoint-server-2007-search-and-sqlfts.aspx
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http://www.ifilter.org/ (This site has some advertising content on it, but the site does have some interesting links to iFilters)