1. (Modern) Homosexual (usually male). The politically correct term for queer, or one who loves, sexually and romantically, members of their own sex.
2. (Douchebag lingo). Stupid, boring. (i.e. "that's gay.")
3. (Archaic) Happy, joyful.
Note: the second definition is considered highly offensive to LGBT community and their allies. Use of the word 'gay' in this context really does make you a douche.
Person A: I'm gay. I've always been attracted to men, and I have finally decided that there is no point staying in the closet because nothing can change who I am.
Person B: Good for you! :)
Douche A: Look at my new affliction shirt! My old one was so gay, so I'm going to wear this one every day now, for when I'm not at the gym. I can bench 400.
Douche B: Gay. I can bench like 450. Can you take a picture of me so I can show off my spray-tanned abs?
Douche A: Yeah man. I benched like 600 today. Could have done better. Look at me.
Douche B: Lets go take advantage of women so the world can know how straight we are.
Gay guy: I'm gay. You're the first person I've told.
Person who thinks they're the freshest thing since sliced bread: I'm feeling gay too! Let's go and be jolly and gay, and have a gay old time. *Self-satisfied smirk*
Gay guy: Bitch, fuck you.