B1 File Manager is a free file explorer for Android.
The program's simple interface allows you to move/ copy/ delete/ paste/ rename files on local storage, SD cards and USB drives, and there's support for FTP, SFTP and SMB. (SD card handling is more complicated under KitKat/ Android 4.4, but there's a sort-of workaround here.)
The author also produces B1 Free Archiver, and File Manager includes all the same technology, so you can open 38+ archive formats (including multi-volume and password protected) and create zip and b1 archives.
The other highlight is the cloud storage support. Not only can the program handle Dropbox, Google Drive, OneDrive, Box and 4shared, but it also allows you to browse multiple accounts on any service. (And if you don't have multiple accounts, no problem - you can create new ones without leaving the app.)
It's not the most powerful of its type, but B1 File Manager is easy to use, and the cloud storage and archive features are especially strong.