The open-source LPT tool JMeter provides an excelent framework for load- and performance-testing several server types. Downloadable from http://jmeter.apache.org/.
With its full multithreading framework JMeter allows concurrent sampling by many threads and simultaneous sampling of different functions by seperate thread groups.
A GUI allows fast operation and high usability. Test szenarios can be recorded via a proxy server be by replayed offline. It is completely portable and 100 % Java.