Windows: Whether Windows isn't sure how to open a specific file, or you'd rather just use a different program than the default, you probably have to deal with Windows' Open With menu from time to time. OpenWith Enhanced replaces this basic tool with one that tells you which programs are the best options, lets you remove associations, and even change icons for specific file types.
Windows' Open With menu is fine, but it's pretty basic. OpenWith Enhanced gives you a bit more control, recommending certain apps for the job over others, as well as recommending apps you don't yet have installed (and giving you a link to download them). It also adds links to the bottom of the Open With window that let you remove the file association entirely, and edit the "Open With" context menu to remove the clutter. It'll even let you change the icon associated with a specific file type. Sometimes the recommendations are a little off (why would I want to open a PDF with Notepad?), but otherwise it's a useful addition to Windows. It's a really simple install too -- just run the installer and Windows will automatically deliver these enhancements whenever you enter its regular Open With dialog.
OpenWith Enhanced is a free download for Windows only.