Many of my favourites have already been listed...
In particular: Beautiful Thing, Get Real, and L.I.E.
I tend to agree with a few others here regarding Brokeback Mountain. A good enough film, but not as good as all the hype about it suggested.
A few others that (I think) have not been mentioned:-
Wilde: With Stephen Fry. Based on Oscar Wilde's life. A sad story.
Total Eclipse: Based on a true story. The scene where the naked Leonardo DiCaprio (the young poet Rimbaud ) has sex with Davis Thewlis (his older mentor) amazed me when I first saw it.
My Own Private Idaho: Just gotta love River Phoenix and Keanu Reeves. The scene that will always stand out for me is the one where the guys on the covers of the Porno magazines are chatting to each other.
Nine Dead Gay Guys: A hilarious comedy about two Irish lads as they "stumble their way through the London gay underworld in search of gainful employment". I laughed so much my sides were hurting when I watched this one.
The Heart is Deceitful Above All Things: Gay themed, rather than gay, this one is not for the faint-hearted. The child-sexual abuse in this one is far worse than that in L.I.E. I must admit that this one really upset me - even though I had read the book (by J T Leroy) before watching it.
My Beautiful Launderette: Maybe not the best in the genre, but it stands out in my mind as it was probably the first film with a gay theme that I ever saw. It was shown again on Irish TV a few years back.
I must dig through my old video collection sometime. I think there's a few others I have forgotten...
Marty