act vs Octokit: What are the differences?
What is act? Run your GitHub Actions locally. Rather than having to commit/push every time you want test out the changes you are making to your .github/workflows/ files (or for any changes to embedded GitHub actions), you can use this tool to run the actions locally. The environment variables and filesystem are all configured to match what GitHub provides.
What is Octokit? GitHub API Client Library for .NET. It is a client library targeting .NET 4.5 and above that provides an easy way to interact with the GitHub API.
act and Octokit can be primarily classified as "Tools for GitHub" tools.
Octokit is an open source tool with 1.85K GitHub stars and 898 GitHub forks. Here's a link to Octokit's open source repository on GitHub.