Below are some useful links for dealing with depression and suicidal thoughts.

This list is mostly UK centric but also includes some links for international readers. Feel free to add your own suggestions in the comments section.

NHS - Help for suicidal thoughts
https://www.nhs.uk/conditions/suicide/#

International Bipolar Foundation - List of International Suicide Hotlines 

http://ibpf.org/resource/list-international-suicide-hotlines

Samaritans 
https://www.samaritans.org/

Mind
https://www.mind.org.uk/

CALM - Campaign Against Living Miserably 
https://www.thecalmzone.net/

YoungMinds
https://youngminds.org.uk/

Papyrus
https://papyrus-uk.org/

Students Against Depression
https://www.studentsagainstdepression.org/

Suicide Prevention Line (USA)
https://suicidepreventionlifeline.org/

DepressionHurts (Canada)
http://depressionhurts.ca/en/default.aspx

Beyond Blue (Australia)
https://www.beyondblue.org.au/

Depression.org.nz (New Zealand)
https://depression.org.nz/

South African Depression & Anxiety Group
http://www.sadag.org/