Thanks for recommending k6! Disclaimer: I'm involved in the k6 project, so quite biased :) I think k6 is the best tool available if you're a developer who wants to automate load testing. But I also actually wrote a review article comparing 11 different open source load testing tools - http://blog.loadimpact.com/open-source-load-testing-tool-rev...

Tsung definitely! Harder to use at first, but amazingly powerful

Tsung is very good - fast and w lots of features. Its main drawback, IMO, is the XML-based config/user scenarios.