Please specify what type of testing are you trying to achieve. If it is Functional/Regression testing then you need to use alternatives of Winrunner and QTP. But if it is for Load/Performance testing then you need to use open source alternatives of Loadrunner like OpenSTA and WebLoad.
The Impossible just takes a little longer.
Many as Corey says look at the open source tools as they fit this space also some have been created with agile in mind.
there is also huge value in agile in not useing a tool per se but in looking at using the coding/scripting languages like python, ruby and perl within the development environment.
Also as pointed out by Roshan consider what you want the tool to do and how you want to approach testing in agile so you can select the most effective tools for your context and possible work in the white box dev/test manner with development as a direct part of the agile team.
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