What are the best platforms available for on-demand user testing of your product?

BIG fan of UserTesting. We've used it several times to get feedback on pre-released UI from (important) people that don't know Product Hunt already. Pro tip: Be sure to have another person (ideally someone not on your team) read through the instructions and questions asked of testers. It's easy to confuse testers or ask leading questions when you're so close to your own product.

Get videos of real people speaking their thoughts as they use websites, mobile apps, prototypes, and more.

Great for quick reactions to something and Free!

Invision and UserTesting have partnered to give InVision users a faster, more seamless way to capture insight into their app, website, or prototype.

Over 25,000 early adopters ready to provide you with valuable feedback.

The easiest way to recruit beta testers and collect user feedback for Android, iOS, websites, and tech products.

Slightly different than what you're looking for. But I think you'd find this useful as well. FullStory records everything that your users do and then you can replay it later. So you don't get the voice over/paid tester. But you do get to see how real users experience what you've built. Which I think gives you even more valuable information than a paid tester.

I love the philosophy behind UserTest.io. You'll be able to get the videos from people you test with at an affordable rate.

It's a fraction of the cost(~$9 per video) compared to other services and it also offers a free testing tool if you have access to your own users/participants