When someone is venting, am I supposed to say something?
Usually when someone is venting at me, I feel like I should respond somehow and say something, but I have no idea what that something could/should be. Is it better to just listen or try to comfort them in some way?
If they're venting and don't want advice, just show fellow feeling.
man that blows
why would they think that's a good idea?!
they're such dicks
how did they think you were going to react?!
etc.
You can also ask questions to help the person unload more, or to understand what's made them so mad they need to vent. You're not trying to fix a problem. You're trying to help your friend scream into the void and feel heard and supported. Especially if they don't feel heard at the source of their problem.
For me, one reason that I get frustrated with advisors is that they offer level one of advice when I've already tried levels one - three hundred of solutions. I feel slightly condescended to, and then I have to tactfully explain why their solution isn't feasible. Then I end up feeling like a dick for shooting down their help. Or, I can nod, smile, say thanks, that's a good idea, and walk away still feeling unheard and misunderstood.