I have been using Total Commander ($46, 30-day free trial) since the days it was still called Windows Commander (before Microsoft's legal team made the developer rename it). This Windows Explorer replacement is the one window that is constantly open on my desktop. Whenever I need a file, I instantly reach for it.
In fact, Total Commander's streamlined interface makes traversing directories so fast that when I save a file in some application I often switch to Total Commander, quickly navigate to wherever I want to save it, copy the path and paste it into the application's File Save dialog. It's that much faster than Windows' own dialogs.
Much of Total Commander's power lies in its utter flexibility. I changed the default color scheme to use darker shades; I also changed several of its default shortcut keys, so that copying the current path takes a single keystroke now. You can decide how complex the interface is going to be: Use the default menus, or change them to your liking by adding or removing options. And while Total Commander offers a comprehensive interface for editing the different settings, it also lets you edit the settings file manually if you really want to geek out.
To select all ZIP files in a given folder you need only hit the + key on your number pad and type "*.zip" into the dialog that appears. All ZIP archives are instantly selected, and you can then copy them, delete them, or just see how much space they take in aggregate.
Speaking of ZIP files, Total Commander has built-in support for creating and extracting ZIP archives, as well as TAR, GZ and TGZ archives which are common on Linux and UNIX systems. Using plug-ins and external utilities, Total Commander can work with any number of additional archive formats, such as the excellent 7-Zip format.
Total Commander's old-school dual-pane interface hides tremendous power: Each pane can house multiple tabs, and Total Commander can use background processes for time-consuming operations so that copying large files never makes the application unresponsive. It even has a setting for making a "ding" sound when a lengthy background operation completes, so you'd know it's done. If you're viewing a folder that has very long filenames, a single keystrokes switches to horizontal mode, where the panes are placed on top of each other (rather than side-by-side), affording twice as much space for each filename.
You can use Total Commander for accessing remote file systems, as well: It can access shares across your local network, and also features a built-in FTP client. The FTP client is seamlessly integrated within the interface, so that working with remote servers feels just like you're working on your own computer, with the same familiar interface and keyboard shortcuts.
Finally, Total Commander can save its settings and preferences in an INI file. This means that you can take your carefully constructed configuration with you wherever you go.
To me, Total Commander is absolutely indispensible. It is one of the defining applications for my Windows experience. If you've ever felt the need for a powerful file manager, this is it.