Platform as a Service (PaaS) Bag of Links

Big players:

• Amazon Web Services
• Google App Engine
• Azure (Microsoft)

Most of the other large software vendors also have a cloud story, but only their captive customers care.

Ruby-centric:

• Heroku (Ruby, bought by Salesforce.com)
• Engine Yard (Ruby)

Various others, some of which are more in the Infrastructure as a Service space than Platform providers e.g. hosting plus some Content Deliver Network (CDN) capabilties, and other focussed on specific app building tools and scenarios:

• Rackspace Cloud (Iaas)
• GigaSpaces (Iaas)
• Joyent (a bit of both)
• GridGain (Iaas)
• VMForce (Salesforce.com & VMWare — a bit of both)
• Cordys (app builder)
• WorkXPress (app builder, No Programming Required! right.)
• Wolf (app builder)