Here's a non-exhaustive, un-ordered list of other organisations and individuals whose work in this area we are aware of, respect, and appreciate. Feel free to suggest other groups in the comments below.
Digital Action work mainly behind-the-scenes, to encourage and help organisations to work together to combat digital threats to democracy.
Carnegie UK Trust
The Carnegie UK Trust has led the way in developing ideas around regulatory approaches to Online Harms which focus on systemic regulation rather than relying so heavily on "notice and takedown" of problematic pieces of content. Their work on the idea of an online "Duty of Care", developed by Will Perrin, Lorna Woods, and Maeve Walsh, has been particularly influential.
Glitch, led by the inspirational Seyi Akiwowo, supports those on the receiving end of online abuse through training and workshops, alongside powerful campaigns targeting decision makers.
Centre for Online Civic Culture Loughborough University's Online Civic Culture Centre (O3C), led by Prof. Andrew Chadwick, applies concepts and methods from social science and information science to understand the role of social media in shaping our civic culture. They've produced important research and ideas looking at the relationship between social media and the health of democracy, for example insights into how and why UK social media users share misinformation.
WebRoots Democracy is a young and innovative think tank focused on the intersection of technology and democratic participation. They have generated a range of fresh ideas to regulate social media and tackle online harms.
Centre for the Analysis of Social Media
CASM is a centre housed within UK think tank DEMOS, and looks at tech policy, and the impact of technology on society and politics. CASM research has helped highlight the benefits and potential of social media, as well as providing important insights into online harms.
Civic Signals is a new, US-based start-up looking at what lessons from urban planning could be applied to the future design of social media.
https://www.civicsignals.io/read Hope not Hate
Hope not Hate is dedicated to challenging far-right and fascist extremism. They have been at the forefront of documenting and exposing the operation of extremist groups and the spread of extremist messages online.
The NSPCC is one of the UK's leading children's charities, at the forefront of the fight against child abuse. In recent years they had done important work to highlight the risks posed to children by an under-regulated internet. Their campaign for new laws to regulate what they dubbed "The Wild West Web" have proved influential and helped encourage the government to pursue the Online Harms agenda.
Oxford Internet Institute
The Oxford Internet Institute is a department of Oxford University dedicated to social science and the internet. They've provided important analysis and commentary on online harms, digital democracy and politics, and also provided important training and advice regarding disinformation for civil society organisations.
Institute for Strategic Dialogue
The ISD is an international think tank focused on online disinformation, polarisation and extremism. They contribute important monitoring and analysis of social media trends, alongside policy suggestions.
Centre for Countering Digitial Hate
The CCDH is a UK-based start-up concerned about online intolerance and hate, including racial and religious intolerance, sexism, and homophobia.
Centre on Data Ethics and Innovation
The CDEI is a new UK government body tasked with generating ideas and proposals for governance and regulation for digital technologies. https://www.gov.uk/government/news/cdei-calls-for-overhaul-of-social-media-regulation
The Depolarization project grew out of a course at Stanford University, and explores what is driving polarisation in western societies and what might be effective in reducing polarisation and increasing understanding between people with different points of view.
They also curate an invaluable open list of research into polarisation:
Daily Telegraph online harms campaign The Daily Telegraph has run an influential "duty of care" campaign highlighting problems with Social Media and publicising proposals for regulation