I've been using ES File Explorer for years, but since a while it has become an Ad-spammer, it's unusable for me! I've found a complete, open source, ad-free alternative:
Amaze File Manager:
Overview:
* Open Source, light and smooth
* Based on Material Design guidelines
* Basic features like cut, copy, delete, compress, extract etc. easily accessible
* Work on multiple tabs at same time
* Multiple themes with cool icons
* Navigation drawer for quick navigation
* App Manager to open, backup, or directly uninstall any app
* Quickly access history, access bookmarks or search for any file
* Root explorer for advanced users
* AES Encryption and Decryption of files for security (Jellybean v4.3+)
* Cloud services support (Jellybean v4.3+ / requires additional plug-in)*
* Inbuilt Database Reader, Zip/Rar Reader, Apk Reader, Text Reader
* No ads
* FTP server for wireless transfer between phone and computer
lots more...