Certify The Web allows you to generate and then install an SSL certificate for free! In particular, it makes it possible to secure (httpS) an Exchange server or remote desktop gateways.
Certify The Web is easy to install on Windows Server. Once installed, just follow a wizard to deploy your SSL certificate for free. Certify The Web is fully compatible with IIS (Microsoft's web server functionality).
There are paid certificates that you can obtain from authorized resellers such as Global Sign, for example, Certify The Web relies on the free Let's Encrypt solution and takes care of renewing the certificate automatically.