This is the coolest introduction to cloud computing I’ve found: http://cloudshare.com/
Use for demos, sales, onsite. There’s an assortment of machine configurations, direct access through leased IPs, and base OS’s just waiting to be tapped for whatever purpose. Machines stay active or can be revived from a snapshot taken after configuration. Pro version is free in beta right now, take advantage while you can – up to six machine images simultaneously and networked together, plenty of disk and RAM.