APITools.io has created unparalleled performance testing tools for APIs. It’s free, easy to use, and we’re adding new tools all the time. Review your performance metrics without writing a single line of code. With powerful tools like Reviewer and Draftsman you can A/B edit and review endpoints, making iteration quicker and easier than ever before!
Check out our free and easy to use performance testing tools, and get your APIs and systems under load so you can identify and fix bottlenecks. You can also capture performance metrics for your critical APIs and create test scripts to benchmark latency and throughput before getting started with load testing. With the right tools, you’ll be able to keep your application highly available no matter what traffic gets thrown at it.
LoadNinja
LoadNinja by SmartBear allows you to quickly create scriptless sophisticated load tests, reduce testing time by 50%, replace load emulators with real browsers, and get actionable, brower-based metrics, all at ninja speed. You can easily capture client-side interactions, debug in real time, and identify performance problems immediately. LoadNinja empowers teams to increase their test coverage without sacrificing quality by removing the tedious efforts of dynamic correlation, script translation, and script scrubbing. With LoadNinja, engineers, testers and product teams can focus more on building apps that scale and less on building load testing scripts.
Features:
- Scriptless load test creation & playback with InstaPlay recorder
- Real browser load test execution at scale
- VU Debugger – debug tests in real time
- VU Inspector – manage virtual user activity in real time.
- Hosted on the cloud, no server machine & upkeep required
- Sophisticated browser-based metrics with analytics and reporting features
Protocols:
- HTTP, HTTPS, SAP GUI Web, WebSocket, Java based protocol, Google Web Toolkit, Oracle forms, & more
StresStimulus
StresStimulus targets application scenarios that are difficult to test with other tools. It automatically fixes playback errors thanks to its proprietary autocorrelation. StresStimulus records user actions and replays them to emulate variable usage patterns. It also monitors load impact on application responsiveness and server infrastructure. The result analyzer finds hidden concurrency errors left unexposed by functional testing and includes detailed application performance metrics.
Features:
- End-to-end Test Wizard covers all steps from recording to test results, which reduces the learning curve
- On-premise or cloud testing supported with multiple load generators
- Works as a stand-alone tool or Fiddler add-on
- Exports script to Visual Studio test format to help with scenarios that fail when recorded in Visual Studio directly
- Manual scripting is not required but available; supports three scripting languages
Apache Jmeter
JMeter is an open source tool that can be used for performance and load testing for analyzing and measuring the performance of a variety of services. It is one of the best load testing tools that is mainly used as website load testing tools for web service applications.
Features:
- This free tool doesn’t demand state of the art infrastructure for load testing and supports multiple load injectors managed by a single controller
- It is one of the performance test tools which is highly portable and supports 100% all the Java based apps
- Less scripting efforts as compared to other api performance testing tools because of its user-friendly GUI
- Simple charts and graphs sufficient for analyzing key load related statistics and resource usage monitors.
- Supports Integrated real-time, Tomcat collectors for Monitoring
Protocols
Web : HTTP, HTTPS, WebServices : XML, SOAP, etc., Java based protocols, FTP
Fiddler with BlackWidow and Watcher 
This might seem like an odd combination to have on a performance tool list.
But performance engineering expert Todd DeCapua in a previous PerfGuild conference session recommended using Fiddler with Watcher and BlackWidow to create a quick-start automation testing performance solution.
Fiddler enables you to do many things, but it’s probably best described as a packet capture tool.
While Fiddler may not be considered a load testing solution it does many things that allow you to debug website issues, and with one of its many extensions, you can accomplish even more.
Some things you might want to use Fiddler for:
- Troubleshooting issues with your web application
- Security testing
- Performance evaluations
- Debugging web traffic from most computers and devices
- Many integration features
- Handy for finding performance bottlenecks
- Fiddler is already a pretty popular tool among developers. Many use it for debugging to view the HTTP requests their computer is sending to a service or website.
Watcher is a security add-in for Fiddler which will enable you to get some security results quickly. BlackWidow is a web crawler that gives you the functionality to point it towards a web address and then be able to drill down on results.
For someone who’s just getting started in performance engineering, these three tools working together can provide a great way to get that free look and feel as well as results that one might not otherwise be able to obtain quickly.
Todd actually has a session during a past PerfGuild Online Conference where he gives a demo of this approach.
nGrinder 
nGrinder‘s GitHub page describes it as having been designed to be an enterprise-level performance engineering solution. It was developed to make stress testing easy and to provide a platform that allows you to create, execute, and monitor tests.
Features:
- You can write your tests using Jython or Groovy to create test scenarios and create stress against JVM using multiple agents.
- It can extend tests with customer libraries like jar and py
- Allows you to monitor the state of your performance agents load generation
- Take care of automatically collecting test results from distributed agents after tests
It currently has 1.3k stars on GitHub.
The Grinder
The Grinder is a Java-based framework. It provides you with easy-to-run and -create distributed testing solutions using many load generator machines to capture your end-users response times. So you don’t have to worry about any virtual users restrictions.
- You can perform load testing on any system that has a Java API
- A nice GUI console
- It automatically handles the management of client connections and cookies
LoadView
LoadView is a fully managed, on-demand load testing tool that allows complete hassle-free load and stress testing.
Unlike many other load testing tools, LoadView performs testing in real browsers (not headless phantom browsers), which provides extremely accurate data, closely emulating real users. You only pay for what you use and no contracts are required. LoadView is 100% cloud-based, scalable, and can be deployed in minutes.
Advanced Load Testing Features include Point and Click Scripting, Global Cloud-Based Infrastructure, Real Browser Testing
Conclusion
This is a comprehensive performance test run of the Google Calendar API with results, charts and screenshots. These tools will help you design better APIs, with stress tests for each endpoint to find vulnerabilities. Get it free!