If you haven't tried Amazon EC2's spot instances, you really should. Here's a presentation on how to become a power user.
"Amazon EC2 allows you to bid for and run spare EC2 capacity, known as Spot instances, in a dynamically priced market. On average, customers save 80% to 90% compared to On Demand prices by using Spot instances. Achieving these savings has historically required time and effort to find the best deals while managing compute capacity as supply and demand fluctuate.
In this session, we dive into best practices and new features that will help you realize immediate cost savings, maximize compute capacity within your budget, and maintain application availability and performance with less up-front or ongoing development effort. Attendees leave with practical knowledge of Spot bidding strategies, market trends, instance selection and benchmarking, and fault-tolerant architecture with examples taken from common Spot use cases such as web services, big data/analytics, media processing, and continuous integration workloads."