Stunnel is reported to work on:

• Free / Open Source / Community Projects
  • FreeBSD
  • Cygwin
  • GNU/Hurd
  • GNU/Linux (numerous distributions)
  • NetBSD
  • OpenBSD
  • Plan 9 from Bell Labs
• Apple
  • iOS (with Cydia)
  • OS X
• BlackBerry
  • QNX
• Google
  • Android (CLI only)
• HP
  • HP-UX
  • NonStop™ Kernel
  • OpenVMS
  • Tru64 / Digital Unix / OSF/1
• IBM
  • AIX
  • IBM i / i5/OS / OS/400
• Microsoft (32-bit and 64-bit editions)
  • Windows Server 2019 / 2016 / 2012 / 2008 / 2003 / 2000
  • Windows 10 / 8.1 / 8 / 7 / Vista / XP
  • Windows Embedded / Windows CE
  • Windows Phone / Windows Mobile
• Oracle
  • Solaris (also with OpenCSW)
• SCO
  • OpenServer

Please let me know about others so I can update this list.