Looks like Amazon is keen to promote sales of additional software through its Amazon Web Services (AWS) Marketplace — you can now test many of the products offered through the cloud service app store with a time-limited free trial.
AWS announced the evaluation option in a blog post this morning, and says that more than 100 products are on offer.
As with any limited trial, you need to ensure you cancel before the trial period expires. AWS will automatically convert you to an hourly paying customer if you don't. The length of the trial varies (it's set by the developer).