Windows only: The built-in tool for dealing with unknown file types in Windows simply doesn't work. Openwith.org does, and it points you, or your less free-software-savvy friends, to downloads that fit the bill. If you've ever received an urgent email reply (or phone call, or text message) from a parent, friend, co-worker, or anyone else who needs "HELP!" because "this file won't open when I double-click!"—you've turned to the right download. Openwith.org installs an option on the right-click menu for files without an icon and associated program, reading "Openwith.org - How do I open this?" Choose that option, and the Openwith app launches, showing a brief file type description at the top and offering links to download free applications to handle that file. If one is already installed on the system, but maybe not the default handler, Openwith.org knows that and offers to open the file with it.
If the software-less user hits the "Download" link on any choice, Openwith automatically grabs the latest version of the installer for that app. Unfortunately, it doesn't automatically install that app, as most have licenses you have to pretend to read through. Assuming you can explain that process, though, Openwith.org is a neat learning and helping tool for those (poor, unfortunate) souls who don't read Lifehacker on the daily, as well as a useful reminder for the savvier crowd of the options they have. Openwith doesn't use any resources when it isn't running, but if you would rather not (or can't) install the app, its web site and search-able file index does just fine for those one-off extensions you can't quite figure out. For those whose computers aren't quite set up for .mkv videos or .xcf images, it's a great install. Openwith is a free download for Windows systems only.