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There are simply too many tools and too many times this vauge question has been asked and still there is no single answer as there are multiple types of testing - We simply don't know which type you're looking to do.
We all wish there was "one tool" that was "good enough" but at this point it's still an open world.
Of all the software product markets out there, testing probably has the least functional open source offerings. (Compared to web servers, app servers, xml libs, network libs and so on).
Good luck, read - And I'm pretty comfortable saying that testing learning curves are pretty tough because testing is about "evaluation". An automated tool can only collect information, it's up to a human to really determine what it means.
If you've already found a couple of tools of interest (using google or something) we might be able to discuss what you liked/didn't like about them more easily.
Also keep in mind that all tools - if you do not know them - have a learning curve. The actual learning curve for a tool is more dependent on the people who will be using the tool, their skills and capacity for learning than on the tool itself.
I have not failed. I've just found 10,000 ways that won't work" --Thomas Edison