OpenXPKI is described as 'software stack that provides all necessary components to manage keys and certificates primarily based on the X509v3 cryptography standard'. There are nine alternatives to OpenXPKI for a variety of platforms, including Windows, Linux, Online / Web-based, Self-Hosted solutions and Mac. The best alternative is xca - X Certificate and Key management, which is both free and Open Source. Other great apps like OpenXPKI are Smallstep Certificates (Free, Open Source), EJBCA (Freemium, Open Source), VeriSign (Paid) and CertHat - Tools for Microsoft PKI (Paid).
X Certificate and Key management is an interface for managing asymetric keys like RSA or DSA. It is intended as a small CA for creation and signing certificates. It uses the OpenSSL library for the cryptographic operations.
PKI solution based on Microsoft AD CS (Active Directory Certificate Services) offers a affordable and powerful digital certificates environment, however sometimes it can be challenging to use without additional tools (3rd party utilities or scripts).
The Dogtag Certificate System is an enterprise-class open source Certificate Authority (CA). It supports all aspects of certificate lifecycle management, including key archival, OCSP and smartcard management, and much more.