Just finished a installation for a customer. After thinking a bit I decided that in addition to the out-of-the-box provided iFilters the following are quite usefull and should realy cover about 90% of files. Remember that all this indexing also adds to the load of the indexer… 

A great suprise in the Office 2007 pack is the support for zip-files – I have seen some realy expensive iFilters for that.. 
Office 2007  (.docx, .docm, .pptx, .pptm, .xlsx, .xlsm, .xlsb, .zip, .one, .vdx, .vsd, .vss, .vst, .vdx, .vsx, and .vtx): http://www.microsoft.com/downloads/details.aspx?FamilyId=60C92A37-719C-4077-B5C6-CAC34F4227CC&displaylang=en

XPS havent realy had its breaktrough yet, but the iFilter will be increasingly more sought after..
Microsoft XPS (.xps): http://www.microsoft.com/downloads/details.aspx?familyid=b8dcffdd-e3a5-44cc-8021-7649fd37ffee&displaylang=en&tm

Adobe have now released a x64 filter for PDF which finaly makes us be able to remove the FoxIT PDF iFilter if you use x64 which you should..
Adobe PDF x64 (.pdf): http://labs.adobe.com/wiki/index.php/PDF_iFilter_8_-_64-bit_Support