Free, simple yet robust and reliable SSH(2) and SFTP Server for Windows. It features 3 authentication types (including public-key auth with multiple keys per user) as well as a good SFTP subsystem and channel forwarding.

And it has some nice security features such as a forced delay upon new connection (useful to defeat brute-force attacks) and a handy "tarpit" to ban attackers´ IPs.

Also, it fully supports virtual folders with per-folder security. And its remote management console allows you to manage SilverSHielD from anywhere. Furthermore, you can use the Command-Line Interface (CLI) to remotely configure your SilverSHielD server and integrate its management into 3rd party management tools.

Free of charge if 3 simultaneous connections from a single client is sufficient for you.