Khadija - I think it is great to collect lists of tools - but to choose just one to share is a challenge- a teacher might have many favorites because of how they use it. For instance check out Shell's discussion on Recommendations for presentations she got a lot of responses.
Since I am teaching an online course I really like using Remind this is a way I can remind my students through text messages when assignments are due. It helps having another source to get them to where they need to go.
What tool is your favorite and how do you use it with your students?