The OpenOffice program I listed is a replacement for Microsoft Office, it will open and save word, excel, etc files to some extent but since Microsoft keeps the format hidden and keeps changing it the reverse engineering isn't perfect.
I keep with OpenOffice because I can legally install it on someone else's computer since there are not licenses to be purchased.
The difference between a free app like openoffice and one like the planner you mentioned is the large open source projects like openoffice are not owned by one person or company. Those small person owned or company owned apps might not be available tomorrow but open source apps are worked on by many so if one programmer stops helping there are many others that still help and new people joining all the time so its a safer bet that it will be around for a long time.