Ideally, during the design phase of a project, you will receive important performance criteria such as the target number of simultaneous users the system should support and the hardware the system will run on. Of course, we don't always live in an ideal world, but you will still be responsible for delivering an application that meets all performance criteria. Don't wait until the end of the project to begin measuring performance.
You should build tracer bullets for the system incrementally ”single slices of business value that utilize major pieces of your expected final architecture ”and then measure performance of those tracer bullets. If you don't, expect to work some late hours and weekends the first few weeks after product launch. The faster you can identify potential bottlenecks, the quicker you can re-architect and avoid them.
JMeter is distributed by The Apache Software Foundation, under The Apache Software License, Version 1.1, at:
http://jakarta.apache.org/site/binindex.cgi