Lucidchart is a great online tool that you can use to make a wide variety of diagrams including concept maps and venn diagrams.  They have a lot of templates to get you started.  Lucidchart can also be integrated with your Google Drive and you can even create new diagrams straight from your Drive account.  This also allows you to create while offline.  Since these diagrams and charts are created in your Drive, they can be shared and worked on collaboratively just like any other Drive file.