Load testing tool
I'm looking for a load testing tool that supports 20+ virtual users in trial.
Ambiguous question, but check The Grinder out
What environment? you need to be more specific. If it's a simple server environment? do you need to use physical users or can you use something like Apache JMeter?
Apache JMeter should be good enough to run fast. However, I found that Webload (only supported on Windows) is a more complex and more convenient option for tests that run for a longer time. I continued to use both of them depending on my mood swings.
Prasanna S
I once used Apache jMeter for my thesis. With its load balancing capability and its intuitive and powerful gui configuration, you can simulate many "complex" users using many services limited only by computing power (like the number of computers you control), I would also recommend looking at it.
This answer only helps if you are looking for a web testing tool.
You can get 25 virtual users for free at http://browsermob.com . This is a different type of load test. It uses real browsers to replay every transaction, which means your scripts can be much simpler and the traffic generated is much more realistic.