Several sites are posting these, but bbc has an article with some friends' comments on Heath Ledger... here are a few.
"Heath has touched so many people on so many different levels during his short life but few had the pleasure of truly knowing him. He was a down to earth, generous, kind-hearted, life-loving, unselfish individual who was extremely inspirational to many."
Heath Ledger's father Kim
"It is tragic that we have lost one of our nation's finest actors in the prime of his life. Heath Ledger's diverse and challenging roles will be remembered as some of the great performances by an Australian actor."
Australian Prime Minister Kevin Rudd
"Working with Heath was one of the purest joys of my life. He brought to the role of Ennis more than any of us could have imagined - a thirst for life, for love, and for truth, and a vulnerability that made everyone who knew him love him. His death is heartbreaking."
Brokeback Mountain director Ang Lee
"I adored him. I don't know how to compare his talent to others but he's touched me deeply as a talent and it's a great loss - losing him at any age would be a loss but it was pretty rough news. I was really shocked by it."
"I had such great hope for him. He was just taking off and to lose his life at such a young age is a tragic loss. My thoughts and prayers are with him and his family."
"What a terrible tragedy. My heart goes out to Heath's family."
"I am shocked and very saddened by the news. I deeply respected Heath's work and always admired his continuing development as an artist. My thoughts are with his family and close friends."
"I think Tennyson got it right in the poem when he described someone as having died at a young age but burning the candles at both ends, and oh what a beautiful flame he made, that was Heath, what a beautiful flame he made and a great talent. My heart goes out to everyone in his family and my family. The saddest thing is his daughter whom he just loved dearly."
Larry Williams - ex-partner Michelle Williams' father
And, to remember him in my own way, here he is in (to me) his most memorable role:
I'm going to be done talking about this after this posting, because there's only so much that can be said before it becomes speculation, which I see as being pretty disrespectful considering that I did not know Heath personally and have no grounds on which to judge him, and while that never stops me, it does in this case because this whole thing is really sad. Anyways... it's clear that he was well respected by his colleagues, peers and friends. He was a phenomenal actor who was clearly a very devoted father to Matilda. I find it unfortunate and a little atrocious that there is video on the internet of his body being brought out of the building - sometimes a line needs to be drawn. So... again, RIP Heath, and true condolences to all who knew him well.