CI/CD Platforms & Tools
Terrateam
GitOps-first open-source Infrastructure as Code automation for Terraform, OpenTofu, Terragrunt, CDKTF, and Pulumi. 
styleci.io
Public GitHub repositories only
DeployBot
REST API to trigger deployments from GitHub, GitLab, and Bitbucket
CodeMagic
Codemagic is a fully hosted and managed CI/CD for mobile apps. You can build, test, and deploy with a GUI-based CI/CD tool. The free tier offers 500 free minutes/month and a Mac Mini instance with 2.3 GHz and 8 GB of RAM
Gitpod
Automated cloud dev environments. 
DatoCMS
Offers free tier for small projects. DatoCMS is a GraphQL-based CMS. On the lower tier, you have 100k/month calls
codefresh.io
Free-for-Life plan: 1 build, one environment, shared servers, unlimited public repos
appveyor.com
CD service for Windows, free for Open Source
bitrise.io
A CI/CD for mobile apps, native or hybrid. With 200 free builds/month 10 min build time and two team members. OSS projects get 45 min build time, +1 concurrency and unlimited team size
ElegantCMS
Headless CMS builder
appcircle.io
An enterprise-grade mobile DevOps platform that automates the build, test, and publish store of mobile apps for faster, efficient release cycle. Free for 30 minutes max build time per build, 20 monthly builds and 1 concurrent build
buddy.works
A CI/CD with five free projects and one concurrent run (120 executions/month)