Policyinstitute.net maintains a database of counter-radicalization and counter-extremism resources. What follows is a shortlist of some of the most important P/CVE organizations and their websites:

Radicalisation Awareness Network (RAN):
https://ec.europa.eu/home-affairs/what-we-do/networks/radicalisation_awareness_network/

Strong Cities Network:
https://strongcitiesnetwork.org/en/

International Centre for Counter-Terrorism:
https://icct.nl/topic/countering-violent-extremism/

Center for the Prevention of Radicalization Leading to Violence:
https://info-radical.org/en/

moonshot:
http://moonshotcve.com/

VOX-Pol:
https://www.voxpol.eu

Search for Common Ground:
https://www.sfcg.org

Impact Europe:
http://impacteurope.eu/

The International Centre for the Study of Radicalisation and Political Violence (ICSR):
https://icsr.info/

International Center for the Study of Violent Extremism (ICSVE):
http://www.icsve.org

Institute for Strategic Dialog (ISD):
https://www.isdglobal.org/

Radicalisation Research:
https://www.radicalisationresearch.org/

German Institute on Radicalization and De-Radicalization Studies:
http://girds.org/

Violence Prevention Network:
http://www.violence-prevention-network.de

Deutscher Präventionstag (DPT):
http://www.praeventionstag.de/nano.cms/

Exit Deutschland:
https://www.exit-deutschland.de/english/

Creative Associates:
https://www.creativeassociatesinternational.com/cve-and-pve/

The Stabilisation Network:
https://www.stabilisation.org/services/

European Eye on Radicalization:
https://eeradicalization.com/

Global Center for Combating Extremist Ideology:
https://etidal.org/en/home/

1st NAEF:
http://www.1naef.com

Hedayah countering violent extremism:
http://www.hedayahcenter.org/

Global Counter-Terrorism Forum, Violent Extremism:
http://www.hedayah.ae/

Global Center on Cooperative Security:
https://www.globalcenter.org/

The Prevention Project:
http://organizingagainstve.org/

rusi.org:
https://rusi.org/CVE/