Of course there are plenty of PM tools on Google Apps Marketplace. One can easily notice that it includes simple To-Do list software to fairly complex PM tools. For project managers, looking for reasonably useful project management tool, he/she need to search with combination of different keywords like Gantt Chart, Agile, Issue tracker, Timesheet Tracker beside task management.

Why should your team access different systems to manage project activities like updating tasks status/progress, creating new issue/bug, raising risks, updating timesheet entries, etc? I would like to suggest ZilicusPM here. http://www.zilicus.com one integrated tool to manage project activities be it tasks, issues, risks, timesheet or even project documents.

I'd certainly recommend ZilicusPM http://www.zilicus.com as a comprehensive project management software.