Hi,
I'm Christopher and my company Webprofusion develops and supports Certify The Web (https://certifytheweb.com) full-time. This app is an extensive UI and background service for managing certificate requests, renewals and deployments via ACME (Let's Encrypt, BuyPass GO, ZeroSSL etc). The app is currently used on Windows servers but is expanding to Linux for centralized certificate management scenarios. The app was originally focused on IIS when we started in December 2015 but has grown to support a wide range of other certificate deployments (which can be customized). We try to cater for all levels of user experience and all common certificate uses.
Our software has a free Community Edition which a large part of our user base use, this manages a limited number of certificates and is otherwise unrestricted, support is provided via https://community.certifytheweb.com/ - we currently have more than 100,000 daily users.
We also offer Starter Edition ($49 for 12 months license, 1 install), Professional ($99 for 18-month license, 3 installs), Power Pro and Enterprise licenses of our software which are just limited on the number of installs for each tier. These versions come with access to our support helpdesk ticketing system - this (combined with app ease-of-use) is where most customers get extra value from our service. Many of our customers are not especially confident with managing and using certificates so they get a real benefit from having someone they can ask and expect a reply from.
Some of our customers operate the software on more than 100 servers, and some manage certs for more than 2000 domains per server and they get visibility on renewals and also use our centralised reporting dashboard feature.