The sites on this list are those which I have found personally useful and can recommend.

Great War Forum - quite simply the largest Great War forum with 27,000 members and in excess of 1,000,000 posts.

Long Long Trail - "The Long, Long Trail is all about the British Army in the First World War:
aimed at the family and military researcher, it is a tribute to the men and women
who fought and won - and to the million who died trying
" - created by Chris Baker, past president of the Western Front Association & the creator of the Great War Forum.

Western Front Association - "Established in 1980, the Association has grown over the years to more than 6,500 members worldwide.  The WFA supports many remembrance and research projects, from the renovation of battlefield memorials, to organising care for the veterans, to re-establishing the 11 o'clock two-minute silence at the Cenotaph on 11 November each year" -  also home to the WFA's Front Forum

Inst in the Great War - "Over 700 old boys of the Royal Belfast Academical Institution, commonly known as Inst, served in the First World War, and over 130 of them died. This site attempts to tell the story of those who did not return"  - created by Alan Curragh.

Centre for First World War Studies - University of Birmingham - the Centre was launched in 2002 and offers a BA iin War Studies and MA's in First & Second World War Studies.

Passchendaele Remembered - one of military historian Paul Reed's sites which focuses on the 1917 battle for Passchendaele.  The site also hosts Paul's Ypres Forum

Canadian Expeditionary Force Study Group - an excellent forum dedicated to the study of all aspects of the Canadian Expeditionary  Force.