As the title says, I am seeking a project planner with automatic resource levelling. It must be free for commercial use and a bonus, but not requirement, would be MS project import/export.

I like the look of Task Juggler, but it is at a stage of development hovering between v2 and v3.

Anything else? Basically, I want to play "what if games" - I will determine the tasks and their effort, and the dependencies between them and then try to figure out how many staff I need.

Since many of the tasks can be done in parallel, it is difficult to guess how many are needed. A PM tool with automatic resource levelling seems like a good way to find out.