Good overview of various clients (some are below):
http://www.techsupportalert.com/best-free-ftp-client.htm
FileZilla
From the site:
FileZilla Client is a fast and reliable cross-platform FTP, FTPS and SFTP client with lots of useful features and an intuitive graphical user interface.
main site:
http://filezilla-project.org/index.php
Latest install used by me is attached below (add .exe to the filename after download). Newer version is sure to be available; support for the open source software is ongoing; option in program to check for updates.
SFTP Net Drive - by EldoS Corporation (Not open source, but Free to use.)
http://www.eldos.com/sftp-net-drive
From the site:
With SFTP Net Drive you can work with the remote file system as if it were a local disk drive, given that the remote system supports SFTP protocol.
Direct download:
SFTP Net Drive Free - Version 1.0.19 (5668 Kb) . Released on 10 June 2012.
SFTP Net Drive for Windows XP, 2003 Server, Vista, 2008 Server, Windows 7, 2008 Server R2. 32-bit and 64-bit platforms are supported.
ExpanDrive
** Allows drive mapping in Windows; $40
Swish (Free)