HashiCorp Vault - is big in Secrets management but its not the Terraform of Secrets management it's just too easy to make another product.

AWS Secrets Manager

AWS Systems Manager Parameter Store

AWS KMS

Google Cloud KMS - Cloud Key Management System

Azure Key Vault

confidant - https://lyft.github.io/confidant

keywhiz - https://github.com/square/keywhiz

knox - https://github.com/pinterest/knox

strongboxsafe - https://strongboxsafe.com

conjur - https://www.conjur.org

+ many more

Ansible Vault - probably not competing directly

I am pretty sure companies like netflix, facebook, uber, tesla and others are probably using their own in house creations.