This seems to be an older thread, but I'm updating my dbase of collab apps as well. If you haven't already, after finalizing your functional requirements, consider looing at OpenAir (www.openair.com) and QuickBase (www.quickbase.com). These are more akin to PSA (professional service automation) tools that link into accounting systems, etc. But if you have to manage freelancers and billable hours against projects, etc. PSA tools have more legs and will take you farther. There are also good old standbys like eRoom (now Documentum eRoom). At the end of the day, it's your functional & business requirements that will rule your decision.

My personal favorite at the time of this writing is Copper Project. I'm finding more and more that the Basecamp metaphor is too soft and too quasi-agile. I am hearing complaints from team members at various levels about its inability to provide enough command and control over the visualization of tasks and milestones. And despite the benefits of true Agile (which is practiced less often than claimed) team members/management/key actors often crave dashboard Gantt or Gantt-type views of project status, cascading dependencies, etc. So, think about your needs beyond simple task list management before you make your decision.

And good luck!