http://www.virtualization.info/radar/ : Categorized list of companies, cloud enablers
http://www.3tera.com/AppLogic/ : 3Tera's platform for cloud and utility computing; available as a service or in-house license
http://aws.amazon.com/ : Amazon web services (EC2, S2, SQS, etc.)
http://www.cassatt.com : Platform for enterprise clouds (using existing data center resources)
http://eucalyptus.cs.ucsb.edu/ : EUCALYPTUS: open-source software infrastructure for implementing "cloud computing"; API compatible with Amazon EC2
http://appengine.google.com : Google AppEngine (PaaS)
http://developer.intuit.com/quickbase/ : Intuit QuickBase (PaaS)
http://www.mesh.com : Microsoft Mesh (PaaS)
http://www.mosso.com : Rackspace's hosting cloud
Virtual Workspaces project: open source (Apache2 licensed) cloud and grid computing toolkit for existing resources
http://www.grid.org.il/?CategoryID=497 IGT Cloud Computing Knowledge Base
Cloud Computing Portal
ReliaCloud : One of the nation's largest cloud computing platforms. Complimentary load balancing and high availability in servers.
GoGrid - offering "Control in the Cloud"
CloudStatus : Realtime performance monitoring of clouds (currently AWS), follow performance on Twitter
UtilityStatus : Application monitoring as a service, deep dive diagnositcs for Java applications running on the cloud.
ControlTier Deployment Automation : open source tools and automation framework for orchestrating the deployment of complex applications to local and cloud infrastructure.
Cloud computing tutorial has articles on cloud computing.
CloudSleuth.net: Visualizing the user experience of leading cloud providers.