I am working on load testing a fully developed web application. I want to test the load it can take (by creating virtual users), test the stress it can take( by going beyond the point it can take and stressing it till it breaks), volume test(where Huge volumes of data are virtually created and determine the DB stability)
which open source tool can do all the 3 effectively
OpenSTA, JMeter, and The Grinder are all good choices for open source tools. The link Tinker provided is the way to go. I personally prefer The Grinder, then JMeter, then OpenSTA, but your mileage may vary.
If you're willing to pay for a low-cost service, I recommend you look at my new company, BrowserMob:
BrowserMob is based on Selenium, an open source functional testing tool. Instead of simulating HTTP traffic, it actually runs hundreds or thousands of dedicated real browsers. This makes scripting really simple and ensures an authentic load profile.
Founder, OpenQA and Selenium Remote Control
Founder, BrowserMob: On-demand, low-cost load testing with real browsers
<ul type="square">[*]On-demand: Control thousands of browsers in minutes.[*]Uses real browsers: True no-hassle scripting and automatic AJAX/Flash support.[*]Great reporting: MySQL data dump, full object-detail, real screenshots of browsers.[*]Low cost: Run for as little as $1 per virtual user per hour.[/list]