I have been evaluating KpyM SSH Server and it looks pretty good and is BSD-licensed with source. It is based on cryptlib fundementally and creates a Window Station for each session which is why it is one of the few open source SSH solutions that works correctly with PowerShell.
We have been using OpenSSH compiled for Interix/SFU/SUA for the last several years. The downside is that OpenSSH doesn't play all that nicely with PowerShell which is an annoyance and it requires the full POSIX subsystem from Services for Unix 3.5 or the Subsystem for Unix Applications (Win2k3 R2 and later).
Years ago we used to use Vandyke and it worked well.
Bitvise WinSSHD is very nice. Supports aes256 and aes128 out of the box. It is not open source but it is free (with AD integration crippled) for personal use and very reasonable $100 USD per server for commercial use. Can be configured to use powershell as the default shell and powershell works correctly. WinSSHD has very granular configuration per-account and per-group and per client IP and per client DNS. There are logon and logoff actions that can be configured per account or group. Supports OpenSSH public key files. Exposes an automation API. Write logs to the Windows event log and/or text file. Still has a small and light service process.