With LoadImpact.com they take care of the software and servers distributed globally for generating the load. You just pay for the pleasure! Realistically if you are load testing you want to generate substantial load. There lowest package for load testing a site that may get 10,000 visitors a month you can do for free. But to generate load to test your server you’ll really want their next package with is equivalent to getting 100,000 visitors a month and costs $225! Now if you’re planning a highly popular website you’ll probably be pushing 1,000,000 visitors a month, well that’s going to cost you $2,100!! It’s a great service but I can’t justify spending that every month on load testing (and you’ll probably have to pay for bandwidth charges through your web host so that’s even more cost).
webperformance.com have a paid (pro) service that also costs quite a bit but they also have a lite option where all you do is register and then download the software. With these guys you install the software on one of your own servers (or a PC somewhere) and then run and configure the load test your self. The advantage here is you can run the load tests from a server within your network which means in most cases there is no bandwidth charge. If you want to test some serious numbers or just want to distribute your load test over a number of servers you can either distribute this over the cloud using Amazon EC2 (for a nominal charge) or you can install the load engine component on you’re own servers in any data center.
Even the lite option is good for 1,000,000 concurrent users! You would need a mortgage to achieve that with LoadImpact.
To configure the load test you basically browse your site and the software keeps track of everything you download and access. Then all of those actions are replayed during the load test.
There are some limits to the lite option but it’s a pretty good alternative (I’m not sure if you can use SSL and I definitely know you can’t use ajax).
Here is an example report I exported the other day and saved as a PDF - some items are reserved for the pro version but there is plenty of information there for my needs.