This is a great list. I like Insomnium the best because of it's complete offline functionality.

But the list can't be complete without mentioning these three noteworthy alternatives, two of which are not GUI based (works far more efficiently for developers). All three are VS Code extensions:

1. Thunder Client (Not really free, but freemium): https://marketplace.visualstudio.com/items?itemName=rangav.vscode-thunder-client

2. zzAPI: Free and open source: https://marketplace.visualstudio.com/items?itemName=AgroStar.zzapi: and GitHub repo: https://github.com/agrostar/zzapi

3. RestClient: (free and open source): https://marketplace.visualstudio.com/items?itemName=humao.rest-client