Ok, everybody's seen movies where the main actors are in a semi-dark room, and there are a few windows that offer the only light. The sunlight streams in from these windows, and you can see the actual beams of light before they fall on the floor, achieving a beautiful and contrasty effect. Does anyone know how this effect is achieved (making the beams of light visible)?

Yes! You need to fill the room with fog. Use a fog machine and let it accumulate until its not swirling anymore, and is a sort of dense fog. Let it dissipate a bit. You'll need really bright light outside the windows, and the right angles and camera settings, but after some experimentation you can make it look totally awesome.

fog machine + 650w (or higher) fresnel works best for me, since you can create a more concentrated beam with the fresnel.

yes fog machine. or if it's not in a place that you don't mind getting dirty, use chalk or dust because the particles reflect very very nicely (fog doesn't really reflect anything, it looks hazy)

especially if there's like insect wings and little bits of reflective material like chips of the reflective film on the back of the mirror, or chrome that has fallen off furniture. very very small. you can't really reproduce it, it's just dusting up around areas that have those things.

you really want to achieve a consistant level of smog, meaning, that there has to be limited movement of air, and the area must be sealed to stop smog excaping otherwise you will not have continuitity.