The MSN Search team recently silently released http://addins.msn.com which among other things provides the API documentation for Windows Desktop Search that ships with the MSN Search Toolbar. The lowdown on what the API is good for is right at the beginning and is excerpted below

Which Extension Technology to Use?

There are two basic methods for creating add-ins for Desktop Search.

1.     Adding new file types by creating IFilters

• Extend Desktop Search with an IFilter add-in that knows how to “crack” the contents of a new file type in order to index its text and metadata.
• To do this, you need to build and register an object supporting the IFilter interface.
• You can add file-specific icons or context-menu handlers by following this documentation on extending the Windows Explorer file types by creating IContextMenu and IExtractIcon interfaces.

2.     Adding a new store by creating protocol handlers

• Extend Desktop Search so that it can index a new data store, such as the database of an e-mail application.
• To do this, you need to build a protocol handler object supporting the ISearchProtocol interface, along with an IUrlAccessor to pull the items. If the contents of the data store are file types not already indexed by Desktop Search, you may also need to implement one or more IFilters..
• To add icons or context-menu handlers, you need to implement portions of an IShellFolder.

You can also find custom IFilters at http://addins.msn.com including ones for PDF, ZIP, CHM and Mozilla Thunderbird mail formats. If only there were C# wrappers for all the gnarly COM interfaces I'd provide one for indexing and searching the cache file format used by RSS Bandit. Then people would be able to search their feeds directly from desktop search. That would be kinda hot.