We recently created a plugin during one of our Hackathons that acts as a To-Do/Reminder system. It's in it's early phases, and there are features being actively added by the community as we speak.

The basics are there: Create, View and Complete Tasks.

Some plans we are thinking about: Sending a To-Do to someone else, UX improvements to make it easier to use common tasks. In the future, we can see this being integrated with our Workflow system, allowing events in Mattermost to create a To-do for users automatically (say, when an incident is logged, create a Todo task for users on call to "Perform a Post-Mortem").

The plugin is not yet available in our Marketplace, but we plan to release it this quarter (by end of March).

If you'd like to check it out, or make some suggestions (log an issue on the repo) - you can find the plugin and development progressing here: https://github.com/mattermost/mattermost-plugin-todo

Ryver has this feature.
I have just moved from ryver to mattermost & I miss the simple task management feature in Ryver!

yes I used an integration module in slack and also I used basecamp and trello for this things, but implementing these little things like Task creation, Task assigning would be great in a team environment.