JMeter is a Java desktop application. Its UI is implemented using the Swing Java API. Due to these two points, JMeter is multiplatform and runs in any kind of machine with a Java virtual machine installed: Windows, Linux, Mac.
Its framework allows concurrency and the execution of several Threads and Threads Groups. This is very useful when trying to configure load and stress tests.
It is extensible and there is a very large list of plugins available.
JMeter is part of the Apache software foundation and is completely free and open source (http://www.apache.org/licenses/).