You've already gotten a plethera of responses, but I thought I'd throw my chip in the pot. My husband has the same issue. Even my female friends at dinner parties get an earful when their husbands seem to wander off to the TV or to check out the newest gadget.
I make a habit of walking away when he starts monopolizing. I'll ask one of the others who seems as tired of hearing everything as I am to refresh our drinks or show me her newest painting, pictures of the kids, anything to escape. When I've been gone for about 5-7 minutes, he'll look for me which pulls him away from the other women and then we'll meander into where the guys are hanging. As he begins anew his diatribe, I slowly wander back into the kitchen or where ever the ladies are and leave him there.
Needless to say, I usually do a lot of wandering back and forth, but everyone gets a break (except me) and when he's looking for me, they get to voice their thoughts.
This was the least rude and non-confrontational method I've used to keep him from monopolizing every conversation. At home, after 30 minutes of listening with no chance for rebuttal, I'd get up from where I was seated and wander out of the room to fix some tea, start some laundry or dry the dishes - just to give myself a rest. He often continued his conversation without me there then came wandering after me to continue his speech. After about two rooms of wandering, he got the point that I'm not participating and then gave me a chance to speak. That's when I pointed out all the wandering I do. I asked if he noticed it - which of course he had. I asked if he knew why I wander and of course he didn't. So point blank with no mercy I told him he does't let me speak so I'm not a participant in any conversation therefore I'm not bound by the rules of etiquette to remain and listen. Then I explain that participating includes being able to speak and be heard which he has not allowed. If he started up again with his bad behavior, I again wandered away. Soon he started recognizing my pattern and as soon as I stood up or walked out, he was right behind me but silent. He'd wait until I had spoken my peace before responding.
He's learning - but it's a slow process to change a bad behavior. It takes consistency. You have to stand your ground and not budge until he is willing to compromise.
We now have great conversations and rarely do I need to wander.