I like to simulate test-clients on an API level (so I like to skip the UI) that act as if e.g. 1000 clients execute tasks at the same time to see how the server stands. So basically I am looking for a tool which helps me to define / scripts to run many API calls at the same time - does anyone know of such a tool?
For 1000 clients I'd prefer Gatling, it can create higher loads with less resources, though it's limited to http request only and you need to develop the scenario in Scala (don't be afraid, there is a recorder that'll help you a lot). Further the reports are nicers.
Apart from that, JMeter - as mentioned by mm759 - will work as well. It's UI is bit more intuitive. Though for 1000 users you might require distributed load generators.