Hi all,
I came across a nice table in a Sharepoint books (Beginning Sharepoint 2007 Administration by Göran Husman) with an overview of the most common Ifilter for Sharepoint and at least one source. This list is very helpful if you are installing and configuring your MOSS 2007 environment adn want to add support for other file formats besides the default ones
Also worth mentionning is that the site www.citeknet.com has a very nice (and free) IFilter Explorer that you can use to see all IFilters installed on your server.