Update 2: I see that this question is getting a lot of views over time. If it did, it seems reasonable that such a firm would advertise for precisely these queries. As my searches for queries for OS X hosting with terms like 'per hour' or 'hourly rates' are turning up very little (basically, just labor fees for hourly repairs), I am inclined to believe that this doesn't exist for some reason.
As has pointed out, there are several good options for that route. Update 1: Just to be clear, I'm referring to services similar to EC2's on-demand or spot instances, where the machine (or virtual machine) is rented per hour, rather than typical web hosting services that involve a monthly subscription. Such a virtual machine service would be very useful for testing software in a reproducible, 'neutral' location. Does such a service exist? (iCloud looks to be just a data storage service, rather than a service allowing remote login, etc.)
Amazon's EC2 service offers a variety of Linux and Windows OS choices, but I haven't found a service offering a similar 'rent by the hour' service for a remote Mac OS X virtual machine.
