The SaaS list: A look at current on-demand favorites

Ask a bunch of enterprise experts what their favorite software as a service providers and you're bound to get a lively debate---and a nice survey of the field. What follows is an list of the SaaS landscape and notable companies to track.
This list was compiled from the Enterprise Irregulars mailing list. The initial question was simple enough: What are your favorite SaaS companies? The only real ground rule was that participants couldn't nominate Salesforce.com---after all the company is already well known. What this list highlights is just how big the SaaS game is. Some of these companies are well known and others are largely unknown. There are also a few outlets from New Zealand, which looks to be a bit of a budding SaaS hotbed.
Now this list isn't comprehensive by any stretch, but it's a handy start. Here's a look by category.
App support:
Billing:
Zuora, which received a lot of mentions. Similar companies include Chargify, Recurly, Aria
Business intelligence, analytics:
Customer insights:
Gravity Research.ly Call centers, helpdesk: Assistly LiveOps Zendesk
Collaboration, community:
CRM and HCM:
BatchBlue SugarCRM Kenexa Taleo RightNow Successfactors Nimble Energy management:
Hara ERP:
NetSuite Plex Online Aplicor Finance:
FinancialForce Intacct FreeAgent Xero Host Analytics Gamification engines:
HR:
Sonar6 Workday Ultimate ADP Rypple.com
Infrastructure:
EngineYard Amazon Web Services
Integration:
Marketing:
Aprimo Cvent Constant Contact HubSpot Eloqua MailChimp Mad Mimi Marketo Extole
Pricing optimization:
Social enterprise:
Spend management:
Ariba Coupa Freshbooks Servicesource
Supply chain:
Utilities: