In any emergency threatening immediate physical harm, call 911.
The U.S.'s top hotlines
Crisis Text Line - for anyone in crisis "anywhere, anytime." Trained crisis counselors respond quickly and chat live in text when someone texts "start" to 741-741.
National Suicide Prevention Lifeline - not just for people in suicidal crisis (if you're aware of someone local who is, call 911). Lifeline operators help via chat (on the home page) or a toll-free call: 800.273.825
Boys Town National Hotline - for girls and parents too. Chat, text or call toll-free 24/7 (800.448.3000) for help in dealing with abuse, anger, depression, school issues, bullying, etc.
National Runaway Safeline - "helping to keep homeless and at-risk youth safe and off the streets." Call 1.800.RUNAWAY (800.786.2929) or click "Chat Live" on the home page. For shelter locations, see this page in the site of its partner Covenant House.
GLBT National Help Center - a Youth Talkline with peer support and information for gay, lesbian, bi-sexual, transgender and questioning young people. Chat live on this page or call 800.246.7743 Mon.-Fri. 4pm-12m ET or Sat. 12n-5pm. There's also a hotline for people of all ages.
The Trevor Lifeline - for 24/7 crisis intervention and suicide prevention for lesbian, gay, bisexual, transgender and questioning youth, call 866.488.7386. Live chat available 3-9pm ET 7 days/week. Or TrevorText is available 4-8pm ET on Fridays – text "trevor" to 202.304.1200.
ReachOut.com - non-emergency support from teens & young adults for teens & young adults facing tough times and struggling with mental health issues – “without fear of being judged or being different.”
Sexual exploitation of children
National Center for Missing & Exploited Children (NCMEC) – for 24-hour help with cases of child sexual abuse (online or offline) or missing children - 800.843.5678
ChildHelp - the 24/7 National Child Abuse Hotline provides help in 170 languages via trained crisis counselors and interpreters. Call 800.4aChild (800.422.4453) or text childhelp to 847411.
Dating abuse / domestic violence
LoveIsRespect.org – 24/7/365 national helpline and info resource for anyone with questions about dating abuse or healthy relationships. Call 866.331.9474, chat in the Web site, or text "loveis" to 22522 (all communication confidential and anonymous).
National Domestic Violence Hotline - for information, support or emergency shelter 24/7/365, call 800.799.SAFE (7233), text "tell now" to 85944 or chat live in the Web site ("compassionate and confidential").
Online hate & harassment
Crash Override Network of the Anti-Online Hate Task Force: a network of former targets of online hate and harassment providing resources, outreach, and support to people being harassed and victimized online. Materials include a "one-page information sheet for employers [including schools and districts] whose employees are facing mob harassment" explaining "how online mob harassment works, what to expect, suggestions for positive courses of action, and some resources for employers who want to dig deeper" and lots of other help at their Resource Center on Tumblr.
National Eating Disorders Helpline - Call Mon.-Thurs. 9am-9pm or Fri. 9am to 5pm ET 800.931.2237
ANAD's Helpline - a service of the National Association of Anorexia Nervosa and Associated Disorders providing information to sufferers and their friends and families, treatment contacts and support group locations. Call 630.577.1330 between 9am and 5pm CT Mon.-Fri. or email email@example.com.
S.A.F.E. Alternatives Information Line - a treatment program, educational resource and professional network of the Self-Injury Foundation founded in 1966. Call 800.366.8288, email firstname.lastname@example.org or fill out the contact form on the Web site.
SAMHSA's Treatment Facility Locator – for 24/7/365 help in finding treatment, call 800.662.HELP (4357) or search the national database for a local treatment center at the Web site of the federal government's Substance Abuse and Mental Health Services Administration.
Revenge Porn/Non-Consensual Image-Sharing
National Human Trafficking Hotline - from the National Human Trafficking Resource Center (NHTRC). Call 888.373.7888 24/7/365.
Digital Security Helpline from AccessNow.org, which "defends and extended the digital rights of users at risk around the world": free "real-time, direct technical assistance and advice to activists, independent media, and [nonprofit] organizations," including info and data security guidance, support, risk-assessment, referrals and training." Email help[at]accessnow.org. See also the "Digital First-Aid Kit."