Openwith.org finds free programs to open unknown file types
It's a simple application that integrates with the Windows right-click context menu. No program available to open a certain file? No problem. Right click on the icon and choose "How do I Open This?" OpenWith.org will open and offer download links for free programs that can handle the file in question.
A massive list of file types are supported and include categories like archives, images, documents, audio files, disk images, and more - there's a complete list on OpenWith.org. It's much larger than the list offered by Microsoft's Application Search, which Windows searches when you tell it to use the web service to find a program. It won't, for example, tell you Firefox is what opens .xpi files whereas OpenWith.org will.
The site is also a nice reference to bookmark for times when you just what, exactly, that file with the .xspf or .xxe extension is.
Openwith.org is a free program, and runs on 32-bit Windows systems only.