The following are the web addresses for individuals in crisis. However, if any person is in immediate danger of suicide, call 9-1-1 (in the US and Canada), or other emergency number in your area, right away. (US, All ages)
This is a GLBT National Help Center website. There both hotlines and talklines listed on the homepage, and also a link to a peer-support chat. (US)
While The Trevor Project can be beneficial to many, it is geared toward our youth, providing crisis intervention and suicide prevention. (US, English & Spanish language)
The National Suicide Prevention Lifeline is also the place to turn to if you are a victim of bullying. There are also resources for veterans. (UK & Ireland)
Samaritans is an emotional support service for anyone in the UK and Ireland, available 24 hours a day.   (U.S.) - suicide prevention, awareness and support  (US & Canada)
Trans Lifeline has a hotline staffed by transgender people, for transgender people.
NOTE:  There have been reports that people have called this lifeline and got no answer.  It is apparently staffed by all volunteers, and the website does indicate when there are no operators on call. 
Suicide survivors forum  --  NOTE:  This forum is not for those who've attempted suicide, or were not successful, but for the people left behind by those who successfully commtted suicide   A listing of suicide and crisis hotlines around the world (International)
Another site for finding helplines and hotlines around the world.  (International)
Listing by country for finding suicide hotlines
ATTENTION:  Because this is an international list, I recommend that you locate the appropriate crisis number in your area in advance, and keep it near your phone or in your contacts to have if ever needed.  A list of this magnitude may not be beneficial if you are in crisis, or trying to help someone in crisis, and needing support NOW.