Jenkins
The leading open source automation server, Jenkins provides hundreds of plugins to support building, deploying and automating any project.
I starting using Jenkins because that is what is used in the new organization I am working. And also the reason I decided to learn about it. I browsed their website, forums, video tutorials (else where off course) and was struck by the simplicity and stability of the tool.
Its amazing what great software gets build when great developers all over the world contribute towards a shared cause, for years! Also I was impressed by their Governance board and formal process of electing and all.
For a little history, it ("Project Hudson") started by one person in 2004 at Sun Microsystems, then Sun got acquired by Oracle, Oracle tried to make it proprietary and the community separated it out into a new fork with todays name "Jenkins"! It is now part of the cd.foundation. They even have portal jenkinsistheway.io for user case studies and stories so everyone can pick a leaf from.
It is absolutely breeze to setup and easy for some basic use cases at least, even for someone who doesn't have any background in build and deploying software.
Worth noting that they do not call themselves just CICD software. It is in-fact an automation server! Anything you can do via command line (and series of commands) can be completely automated with this, CICD happens to be the most common use case.
Its cool. And Free. Check it out.
If you have Pluralsight subscription (not sponsored by them) here are few courses to get you started:
jenkins-2-getting-started (2016)
getting-started-jenkins (2020)