Writing Cross-Platform Node.js - Great tutorial covering many common issues that arise when writing cross-platform code: path creation, script execution, newline characters.
AppVeyor - Focused on Windows. Free tiers are available for OSS projects.
Travis - Windows/macOS/Linux. Free for OSS projects.
Azure Pipelines - Windows/macOS/Linux. Free for OSS projects with 10 parallel jobs.
Github Action - Windows/macOS/Linux. GitHub Actions makes it easy to automate all your software workflows.
Gitlab CI - Windows/macOS/Linux. GitLab CI/CD is a tool built into GitLab for software development.
Virtualization
ievms - Automated installer for the free virtual machine images that Microsoft provides for testing on multiple versions of IE. These images can be useful for cross-platform testing various technologies, however make sure you read and understand Microsofts’ licensing.
VirtualBox - General purpose software for running x86 virtual machines.
Docker - Software platform to create, deploy and manage virtualized application containers on a common operating system, with an ecosystem of allied tools.
Compatibility
Wine - Run Windows API calls on Linux, Mac, BSD and Solaris.