Many vendors of commercial load test tools offer trial versions.
Here are some:
e-Load
http://www.scl.com/oracle/application-quality-management/howtoeval
LoadRunner
https://h10078.www1.hp.com/cda/hpms/display/main/hpms_content.jsp?zn=bto&cp=1-11-126-17^8_4000_100
LoadTest
http://www.appperfect.com/download/index.html
NeoLoad
http://www.neotys.com/evaluation/download.html
OpenLoad
http://www.opendemand.com/openload/trial.shtml
PureLoad
http://www.minq.se/products/pureload/download/
QEngine for Load Testing
http://www.adventnet.com/products/qengine/download.html
Qtest
http://www.quotium.com/form/product_form.php?product=qtest&type=demo
Rational Performance Tester
http://www.ibm.com/developerworks/downloads/r/rpt/index.html?S_TACT=105AGX23&S_CMP=DWNL
SilkPerformer
http://www.microfocus.com/products/SilkPerformer/SilkPerformerProductTrials.asp
SmarteLoad
http://www.smartesoft.com/downloads.php
StressTester
http://www.stresstester.com/stresstester_download.php
TestComplete
http://www.automatedqa.com/downloads/testcomplete/
WAPT
http://www.loadtestingtool.com/download.shtml
WebLOAD
http://www.radview.com/DownloadCenter/ProductEvaluation.aspx
Web Performance Load Tester
http://webperformance.com/sales/signup.html
Webserver Stress Tool
http://www.paessler.com/webstress/download
(last updated December 31, 2010)
