Others have written many things about the great man, I can only add what he and his artistry meant to me:
For me, Leonard Nimoy always embodied the wise and self-controlled paternal figure, whose gentleness was that of a man who was well aware of his own strength and the damage it can cause, so showed restraint wherever possible, but was always prepared to use that great strength to defend the innocent and stand up to evil wherever it appeared. In that his persona was that of that greatest of ancient figures, the warrior poet, who is not only strong and skilled in the arts of war, but also possesses the moral clarity to know the proper time and place to use those arts for the good of those around him and society as a whole. He also embodied the deep thinker, the guardian to another world of wonder and mystery, into which he would allow us to peer ever so briefly and whet our appetite to begin our own search for truth and wisdom.
As Spock he was also the faithful and dependable friend, a brother in arms in times of conflict, the voice of reason and conscience when difficult choices needed to be made and a man struggling to reconcile two different worlds and two different sides to his own nature, struggling to fit into a world where he would always be an outsider. As such he was a kindred spirit and beacon of hope to all of us who never quite fit in with our surroundings, who felt like strangers in our own homes, whose lack of adherence to social superficialities would leave us isolated and alone. Spock showed us that there was a place for us, and not just on the fringes. We could be heroes! More importantly we could be valued, even loved, for who we were.
To every role he played (at least all the ones that I have seen), he brought such a subtle warmth, grace and dignity, it always brought a smile to my face. He was our old friend, here to accompany us on another adventure into the unknown.
He will be sorely missed.