@Task or AtTask is the only collaborative PM tool currently available and has a good, searchable user forum. It is relatively flexible and has considerable functionality. There are some significant bugs, especially in the reporting functions. As of Sept 1, it doesn't interoperate with Salesforce or Outlook, though it's announced to soon. Version 4 can import and export data from MS Project.

Pricing: There is a "Professional" version and an “Enterprise” version; the Enterprise version includes project lifecycle management and capacity planning. The price also includes unlimited phone support and API integration (SOAP).