Microsoft: https://www.microsoft.com/en-us/usability/default.aspx
Salesforce: https://design.secure.force.com/ux/
Google: https://userresearch.google.com/
Atlassian: https://www.atlassian.com/research-group
Adobe: https://www.adobe.com/userresearch.html
Asana: https://asana.com/research
GitLab - https://about.gitlab.com/researchpanel/
Xero - https://www.xero.com/uk/research/signup/
GDS - https://signup.take-part-in-research.service.gov.uk/
Twitter - (invite only 'Insiders' program) https://blog.twitter.com/marketing/en_us/a/2016/introducing-twitter-insiders.html
Amazon - (invite only Amazon Research Panel (ARP) & Amazon Research Club (ARC)) https://www.amazon.com/gp/help/customer/display.html?nodeId=200896290
Archive of post from Research Ops Slack Community - https://researchops-community.slack.com/archives/C9LKA47HR/p1532518619000276 - to avoid losing this when it hits message limit.