Pick 3

 K12 schools have already started or will start very soon so suggesting key online services educators should be familiar with is probably a little late. I agree, but I just encountered this list (50 Web 2.0 sites for schools) and decided tardiness is all relative. By the way, the 50 site list is worth exploring. However, such lists are more suited to the adventurous and those who see their work as making recommendations. For most educators, it is past time to explore and time to focus. Hence, my pick 3 - suggestions for middle and high school educators.

First, I am kind of cheating here because my picks are really services that offer a suite of tools. In my defense, the focus is to consider tools that do not burden users with different connection methods, payment plans, interfaces, but offer versatility and instructional/learning relevance.

1. Zoom - OK, so you may have your own favorite video environment (e.g., Google Meet, Microsoft Teams) and for all practical purposes what you use is a "pick em". I recommend Zoom because it is the service I have actually used in a classroom setting and because it offers versatility - e.g., recording, chat, breakout rooms, access control, show screen).
2. Google Drive - I would describe this as a multi-tool, content creation environment offering opportunities for collaboration, commenting, and revision. It can be used to present content to learns and for learners to create content as a way to process information and develop communication skills. The tools include opportunities for feedback and feedback related revision.
3. InsertLearning - This likely the recommendation most likely to be unfamiliar to educators. It is based in personal assumptions about the use of existing online content as valuable and what I think are needed tools necessary to guide inexperienced learners in processing this content. InsertLearning is one of multiple services that offer what I have taken to describing as "layering" capabilities. What I mean by this is that the educator (and in some ways the learner) can start with an information resource and add elements (highlights, annotations, questions, discussion opportunities) that result in content I think is a more effective educational resource. This tool goes beyond the addition of such elements to include mechanisms for collecting and organizing student engagement with content that can be helpful to educators in making assignments and providing feedback and grading. InsertLearning is specific to web pages. I wish there was a tool that could do layering for both web pages and web videos, but to my knowledge there is not a service of equivalent power for this combination. Other tools do exist for extending online video. See my existing content on multiple layering tools for a deeper description of InsertLearning and tutorials on other services.