What's the best tool to load test my app?

Get monkeys do do it for you 😛 They allow you to do a free test or you can pay to have more monkeys testing your website at once (but at the moment everything's free)!

Postman can help you to (lightly) load test your API for free 😉

Not exactly load testing per-se, but still an interesting solution for API testing!

This is #1 in my opinion. You can record real user scenarios, then run them as percentages of your load test. Example - if you have an ecommerce site, you could simulate 5% of traffic checking out and 95% of traffic viewing product pages. It gets pricy though, so this is best option if you have some budget to spend on testing.

This is my 2nd favorite load testing tool. A bit more basic than Load Impact. But super easy to use and very cost effective. You can customize the types of requests it makes / store reports / rerun old tests. If you're just getting started with load testing, I usually recommend Blitz first.

k6 is a free and open-source load testing tool built for developer happiness.