"Lest we forget" is a poem written by Laurence Binyon. It is commonly heard at British and Commonwealth Remembrance services. In my opintion it is one of the most moving things ever written in the English language:
They shall not grow old, as we that are left grow old,
Age shall not weary them, nor the years condemn.
At the going down of the sun and in the morning,
We shall remember them
I have just found that this is in fact a verse from a longer poem "For the Fallen", but it is only this part that is commonly read on Remembrance Day.