On-line Project Planning s/w

The company I'm working for would like to use an on-line project planning tool, preferably provided as SaaS.  My experience is in the use of MS Project or Project Server, which are installed locally or on a central server.  The main tasks the team wish to manage with the tool are planning activities (i.e. entering tasks, durations, dependencies, and resources), and resource levelling across multiple projects.  The main aim is to be able to plan each project to determine dates for key milestones, critical path analysis, progress monitoring, and the effect of change.

Any recommendation as to which software to use with justification would be most helpful.

Thanks,

Martin