My calendar. As you get away from the development work and into PM work, you get involved in a lot of meetings, and effective time management becomes crucial.
OneNote. I use OneNote as my "external memory", having notes and resource links for all major projects. It magically syncs across all of my devices, so I can access my notes anywhere, any time.
Project management tools. It's important to be able to visualize work-in-progress, so you can have a view of team capacity and project progress.
Communication tools. I talk to a lot of folks inside and outside the company, so I have every possible communication app in existence installed on all of my devices :)