Flipgrid

Difficulty: Moderate

Platform: Web, IOS, Android

Price: Free for basic use; see Classroom Pricing for more features.

Padlet

Difficulty: Easy

Platform: Web, IOS, Android

Price: Free for basic use; Padlet Backpack offers premium features for teachers.

Post-it Plus

Difficulty: Easy

Platform: iOS

Price: Free; the Editor's Pack is offered as an in-app purchase and adds an eraser and extra note colors.

RealtimeBoard

Difficulty: Challenging

Platform: Web, IOS, Android, Google Extension

Price: Free for limited use; a limited version of Pro accounts is offered free to educators.

Slack

Difficulty: Moderate

Platform: Web, IOS, Android

Price: Free for basic use; Pro pricing available.

Skype

Difficulty: Easy

Platform: Web, IOS, Android, and more

Price: Free for basic use features; fees are charged for text messaging and international phone calls, but video calls are free anywhere.

TodaysMeet

Difficulty: Easy

Platform: Web-based. Works on all devices with Internet access.

Price: Free for basic use; $5/month for Teacher Tools.

Trello

Difficulty: Moderate

Platform: Web, iOS, Android

Price: Most features are free, but Trello Gold offers a few more.

Voxer

Difficulty: Easy

Platform: Web, iOS, Android

Price: Free for basic use. $3.99 per user/month for Pro.