sftp

Filezilla – a free and easy to use FTP transfer application

FileZilla is a free, open source FTP client, with versions compatible with Windows, Mac, and Linux. FileZilla supports the use of FTP, SFTP, and FTPS (or FTP over SSL/TLS). This free yet powerful program is useful for regular FTP users who need a good, free option to transfer files. It's simple to setup and run, and it gets the job done. It has a very straightforward and user-friendly interface. It allows you to quickly drag and drop files from your computer to the server, or vice versa. It also allows you to set how many simultaneous uploads or downloads you want to run, making file transfer faster.

Cyberduck

I think Cyberduck is the most user-friendly FTP app out there (too bad it is only for Mac). I was introduced to it a few years ago when I was just barely grasping the concept of FTP and I could connect to different servers & upload files within minutes. Currently the software allows you to browse servers using the following protocols: FTP, FTP/TLS, sFTP, WebDav and browse & upload to Amazon and Mosso's Cloud Storage.

WinSCP

WinSCP is an open source free FTP & sFTP client for Windows. Beyond file transfer, WinSCP offers file management & file synchronization functionality. One of it's biggest advantages its simple user interface - which mimics Windows Explorer (there is one that mimics Norton Commander as well) in both looks and functionality - making a it extremely user-friendly for an avid Windows users. For me, drag-and-drop feature and the ability to see my folder structure is very handy. Even the tool icons look the same as in Windows Explorer, so you will never get lost, even without a manual or a geeky friend next to you.