This may be an obvious hint, but I was playing around with Smart Folders today and figured out that you can make one of the criteria be ANY file type. You are not limited to the ones that Apple provides built-in (such as PDF, Movies, Images).

You don't even need to know the extension, just the plain language that Mac OS X uses to identify file "Kind".

So if you want to make a folder with only certain file types, select "New Smart Folder" from the Finder's FILE menu. You want to search "This Mac" and "Contents". Now click the "+" button to add some search criteria.

Then select "Kind" is "Other". An input field will appear. This is where you type the "Kind" information that is found in the Finder. For example, to search for all Excel files, just type "Microsoft Excel" (this shows you everything, including Excel support files). To search for all Microsoft related file types, just type "Microsoft". You can do the same for "Photoshop" or "Adobe" etc.

Another trick is to whittle it down, like so:

microsoft excel -add-in -library -template -plug-in

(The end result is that you see only file types of "Microsoft Excel Workbook" and "Microsoft Excel Document". I'm not sure what the difference is between a "Workbook" and a "Document" since they both have the extension .xls.)

Very handy when you are trying to figure out where that long lost Excel file is hiding on your computer!

This is, of course, just an example. You can surely think of many ways to extend this hint to help you find or manage the types of files that you deal with on a daily basis.

[And yes, I know you can do the same thing by searching "Name" "contains" and typing "xls", but searching by "Kind" is more useful when you don't know what the extension is, or want to search in ways that are not possible using only file extensions.]