Below you will find a list of organisations and sites that I have found useful or interesting. Although some of the links are only relevant to the UK, I have also tried to include numerous links to prominent Holocaust museums and memorials worldwide (though this is by no means an exhaustive list). All links direct readers to English-language pages, but other languages are available on many websites.
Charities
Facing History and Ourselves
The Holocaust Educational Trust
Holocaust Memorial Day Trust
Former Concentration/Extermination Camps
The Auschwitz-Birkenau State Museum, Poland
Buchenwald Memorial, Germany
Camp Vught National Memorial, The Netherlands
Dachau Concentration Camp Memorial Site, Germany
Gedenkstätte Bergen-Belsen, Germany
Kraków-Płaszów Concentration Camp, Poland (USHMM article - no memorial website)
Jasenovac Memorial Site, Croatia
KZ-Gedenkstätte Flossenbürg, Germany
Klooga Memorials, Estonia
Mauthausen Memorial, Austria
Memorial and Museum Sachsenhausen, Germany
Memorial Ebensee, Austria
Memorial Site Maly Trostenets, Belarus
Memorial to the Victims of the Bogdanovka Extermination Camp, Ukraine
Memorial to the Victims of the Sajmište Concentration Camp, Croatia
Memorial Zone Donja Gradina (formerly part of the Jasenovac camp complex), Bosnia and Herzegovina
Museum - Memorial Site Bełżec, Poland
Muzeum Stutthof w Sztutowie, Poland
The Museum of the Banjica Concentration Camp, Serbia
Museum of the Former Death Camp in Sobibór, Poland
Museum of the Former Extermination Camp in Chełmno-on-Ner, Poland
Oberer Kuhberg Ulm, Documentation Centre and Concentraion Camp Memorial, Germany
Ravensbrück Memorial Site, Germany
The State Museum at Majdanek, Poland
Treblinka Death Camp, Poland (USHMM article - no memorial website)
Warsaw Concentration Camp, Poland (Truth About Camps article - no memorial in existence)
Libraries
The British Library
The Wiener Library
Other Museums
Anne Frank Museum Amsterdam, The Netherlands
Berlin Memorials and Information Centres, Germany
Budapest Holocaust Memorial Centre, Hungary
The Holocaust Exhibition at the Imperial War Museum, UK
Mémorial de la Shoah, France
The National Holocaust Centre and Museum, UK
POLIN Museum of the History of Polish Jews, Poland (not a Holocaust museum, but contains a Holocaust gallery)
Topographie des Terrors, Germany
United States Holocaust Memorial Museum, United States
Yad Vashem, Israel