I've been using Simple Workflow (in particular, the Flow framework: http://aws.amazon.com/swf/details/flow/) a lot recently to manage complex asynchronous distributed workflows, and it's a revelation. Some people laugh about the "Simple" part in the name, but it's kind of true - once you get over the initial learning curve. It's a bit like Git that way (First a lot of banging your head, then a lot of bang for buck)