Directory Opus ($83, 60-day free trial) has a well-deserved reputation as the Swiss army chainsaw of Windows Explorer replacements. Version 9.5 takes this file manager's immense feature set into the Windows 7 era. It's available in 32-bit and 64-bit versions.
Precisely how much Directory Opus replaces Windows Explorer is up to you. When you install it, you can set it to be the default manager for file handling requests, image viewing, and much more--or not. The degree of control which Directory Opus gives the user is extraordinary, especially in an era in which many programs try to simplify or cut down on controls, options, and settings. Directory Opus assumes the user is a grownup and doesn't do a lot of hand-holding or simplification. However, it is rarely confusing or contradictory, just sometimes overwhelming. There are many different ways to do almost anything and many different options to turn on or off (and often you can have multiple windows, each of which may have different values for those options).
As a so-called simple example, you view and manipulate files through what Directory Opus calls a "lister," though most people would just call it "the user interface" or "a file window." A window can show up to two listers, split horizontally or vertically; show a folder tree as well as the folder contents (so if you're viewing two directories, you will also have two folder trees), show a preview pane (which can show most common image, text, and movie formats), and turn on or off multiple toolbars. In addition each of the two listers can contain multiple tabs, each tab showing a different folder and with a different viewing option (detail, tile, thumbnail, and so on), and, simply to add some icing to the cake, you can have multiple lister windows open at once. This is just for looking at files; when you delve into doing things with files, such as batch renames, secure erasure, or filtering, you get even more options. Despite this, you can use Directory Opus "out of the box" with only a little effort as compared to using Windows Explorer, but since it is not a free or even cheap program, if you're going to use it, you should get the most out of it you can.
Among the other things Directory Opus does are FTP support, Flickr integration, ZIP and RAR support, and an API to allow third party developers to add further functionality. (For example, I found a plug-in which added colored syntax highlighting in the viewer pane when I clicked a source code file.)
Of course, nothing is perfect. One thing I would like to do in Directory Opus, but could not do, is to split the viewer pane, so that if I'm viewing two directories at once, I could see a view of two files at once. And while it's not a bug per se, if you remove all the toolbars, it can be unintuitive as to how to get them back (right-click on the menu, choose 'Customize', and you'll see the option you need). Last, all lister windows share the same toolbar sets; it would be nice to hide a toolbar in one window and leave it in another.
Overall, it's hard to imagine a more feature-complete Windows Explorer replacement. At 85 dollars Australian (please check current exchange rates), though, this isn't a casual buy. To get your money's worth, you will need to invest a little time in learning many options and slight tweaks to your default work habits to take advantage of the power Directory Opus gives you.