I have been going nuts looking for a completely free or limited free SFTP server which does not have too many restrictions and is not significantly crippled compared to it's shareware edition. It's amazing how few options there are that fall into this category. I finally settled on the personal edition of Bitvise SSH Server.
Even the personal edition is surprisingly robust and only has a few limitations I can live with.
From the FAQ:
Q000. Where do I get an activation code for personal use?
No activation code is needed to use Bitvise SSH Server for personal use. If your Bitvise SSH Server Control Panel is saying that there is an evaluation
period, this means that you installed the product as the Standard Edition. In this case, you need to uninstall Bitvise SSH Server, re-install it again,
and choose the Personal Edition this time.
Note that Bitvise SSH Server may be installed in the Personal Edition only by genuine, non-commercial personal users who are not using the SSH server as
part of a commercial endeavor, and are not using it in an organization, whether commercial or otherwise. All commercial or organizational use requires
a purchased license.
Q020. What are the limitations of the Personal Edition?
The Bitvise SSH Server Personal Edition:
• On domain member computers, can use only local Windows accounts. On a domain controller, can use domain accounts of that domain controller.
• Can configure only one Windows group (Everyone).
• Can configure only one virtual group.
• Has a limit of 10 Windows account entries.
• Has a limit of 10 virtual account entries.
• GSSAPI authentication is disabled (Kerberos and NTLM).
If you are trying to make a decision about whether to use the Personal or Standard Edition, please note that in most cases, this is not a technical decision.
All organizations, as well as personal users who do not qualify as non-commercial, must purchase a license for the Standard Edition. The Personal Edition
is available only for users who are both personal and non-commercial, and are therefore likely to be unaffected by the above limitations.
Note that this is not a server for the technically faint of heart.