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
DigitalOcean - Simplifying cloud hosting. Deploy an SSD cloud server in 55 seconds.
XCodeClub.com - Rent your own private Virtual Mac with latest XCode.
Amazon EC2 - Amazon Web Services offers reliable, scalable, and inexpensive cloud computing services. Free to join, pay only for what you use.
VirtualMacOSX.com - vmOSX -- is your affordable, Mac-based home on the web.
MacStadium - MacStadium offer Mac colocation and dedicated server rentals plus VMware Mac Pro private cloud environments for large-scale iOS app