IBM Bluemix - IBM Bluemix is an open-standard, cloud-based platform that enables users to build, manage, and run apps on mobile, web, and smart devices.
MacinCloud - MacinCloud is a Mac cloud platform that provides on-demand Mac cloud servers, flexible dev workload infrastructure and comprehensive Mac in the Cloud solutions
Amazon EC2 - Amazon Web Services offers reliable, scalable, and inexpensive cloud computing services. Free to join, pay only for what you use.
XCodeClub.com - Rent your own private Virtual Mac with latest XCode.
Microsoft Azure - Windows Azure and SQL Azure enable you to build, host and scale applications in Microsoft datacenters.
VirtualMacOSX.com - vmOSX -- is your affordable, Mac-based home on the web.