Cubic Explorer is a highly customizable portable file manager
Cubic Explorer has a number of options that have earned it the job. While it's packed with functionality and can be customized any number of ways, the interface remains mostly uncomplicated. It's a given that when you add tabs, breadcrumbs, bookmarks, previewing, folder trees, and everything else normally found in an Explorer replacement that the interface will become cluttered, but Cubic keeps things under control.
I've added all the folders I normally need to access during a repair to the favorites, like c:\windows, control panel, network connections, and my network app shares. After launching Cubic, hitting the bookmarks menu and open all in tabs quickly displays everything. It's much quicker than using start -> run every time I need to open a folder.
Cubic also supports sessions, allowing you to have several customized sets of tabs, bookmarks, and layouts. It's a handy feature for moving between customer systems, our office machines, and my home computers. Several themes are included, and your choice is saved with the session.
Cubic Explorer is freeware for Windows only, and both an installer and portable version are available.