For regressions tests we use soapUI, which seems to be a valid and useful performance / load test tool as well.
However, are there even more advanced load generators especially for web service testing - maybe with reporting features similar to HP LoadRunner?
I like to use Apache JMeter because it's light, it's multi-platform, it supports distributed testing, it can be used for simple to complex scenarii, it has all the "listeners" I need for reporting (most often, I import the results in Excel and then use all the power of Excel). You may find it a bit "rough" compared to HP Load Runner though.
Another famous open source load testing tool is OpenSTA (win32 platforms only). It has nice reporting and graphing features built in and results can also be exported to CSV files. The official web site seems to be currently down.