Windows Certificate Manager Tool (certmgr.msc)
Manage storage for x509 keys. No support for PGP/GPG. Can't sign or encode, can't generate a key. You can use IIS webserver managemnt console to generate a cert.Proprietary
GNOME Seahorse GUI for SSH keys, X509 certs, PGP/GPG. Linux only.
KDE Kleopatra a certificate manager and a universal crypto GUI. It supports managing X.509 and OpenPGP certificates in the GpgSM keybox and retrieving certificates from LDAP servers. You can install it on Windows with gpg4win.
KeyStore Explorer - a GUI replacement for the Java command-line utilities keytool and jarsigner. Written in Java. Windows, Linux, macOS.
Cypt - a nice and clean PGP GUI for macOS. Proprietary
GpgFrontend - a powerful OpenPGP Crypto Tool. Windows, macOS, Linux.
XCA - X Certificate and Key Management. A powerful tool to build PKI. Linux, macOS, Windows.
pyrite GnuPG/OpenSSL encryption/signing GUI for Linux implemented with Python & PyGTK
OpenKeychain OpenPGP for Android. Integrates with K9 mail app and others.
YAOG Windows only. Written in C
OpenSSLUI Windows based simple user interface for creating your CSR or just to create a Self Signed Certificate. Windows only.
Видео Обзор OpenSSL GUI - IIS Windows 10
GUI_OpenSSL. Written in Python with GTK. Ubuntu but may work on Windows.
openssl-wizard A wizard/gui to help you with using the openssl cli. Written in C#. Can be installed on Windows with Chocolatey.
openssl_ca openssl_ca with QT GUI
a7crypt GUI frontend for symmetric encryption/decryption. Bash, Zenity, Linux
pedroalbanese/encryptcli OpenSSL compliant File/Text Encrypter CLI+GUI written in AutoIt3
Dcryptor encryption tool written in C++ using Qt and OpenSSL. It offers a wide range of algorithms and cipher modes to encrypt and decrypt strings, files and folders.
OpenSSL Key and Sign Utility Written in Python
Fortra Open PGP Studio GUI software for verifying, signing, encrypting & decrypting PGP-protected files. Has support for managing a PGP keychain, creating, deleting, import & exporting keypairs. Homepage | Backup of v1.2.2 x64 | Backup of v1.2.1 x86/x64
SignGUI The most reliable frontend for Microsoft's Signtool.exe program (from the Windows SDK Tools). Supports Signtool.exe version 6.3 or later, cans handle SHA2 signatures & dual-signing. Homepage
MGTEK SmartCard Tools An alternative to the Gemalto MiniDriver Manager that supports running Microsoft's Signtool.exe with preconfigured smartcard PINs. Provides both a command-line ScSigntool.exe utility and its smartcard MiniDriver manager. Homepage
PKI Solutions ASN.1 Editor Forensic-grade ASN.1 object inspector, cans handle PKCS#12, PKCS#7, PEM, DER or Hex dumps. Cans also surgically edit certificates & CSR requests, allows arbitrary data modifications. Homepage | Download
P7S Signer screenshot
A proprietary and paid digital certificate GUI for Windows. CAdES, eIDAS, QSCD. Uses .NET
Some more suggestions:
Fortra Open PGP Studio
GUI software for verifying, signing, encrypting & decrypting PGP-protected files.
Has support for managing a PGP keychain, creating, deleting, import & exporting keypairs.
Homepage | Backup of v1.2.2 x64 | Backup of v1.2.1 x86/x64
SignGUI
The most reliable frontend for Microsoft's Signtool.exe program (from the Windows SDK Tools).
Supports Signtool.exe version 6.3 or later, cans handle SHA2 signatures & dual-signing.
Homepage
MGTEK SmartCard Tools
An alternative to the Gemalto MiniDriver Manager that supports running Microsoft's Signtool.exe with preconfigured smartcard PINs.
Provides both a command-line ScSigntool.exe utility and its smartcard MiniDriver manager.
Homepage
PKI Solutions ASN.1 Editor
Forensic-grade ASN.1 object inspector, cans handle PKCS#12, PKCS#7, PEM, DER or Hex dumps.
Cans also surgically edit certificates & CSR requests, allows arbitrary data modifications.
Homepage | Download