I swear one of these times I'm going to type out a lengthy response and save it on my computer so I can just cut and paste when this comes up every year...
A lot depends on what winter conditions are in your area and what tires you run. When I was off at college, snowfall was measured by the foot instead of by the inch, the roads were plowed once a day whether they needed it or not, and they never salted anything. I tried different amounts of weight and different tires with varying results, but the bottom line was that during a good part of the winter up there, the only way my 2wd could get safely around was to run chains - preferably V-bars wrapped around some mud terrains (my lil 2wd Ranger was like a tank running like that, I could do circles around 4x4s).
For mud tires and other aggressive tires, they work great in deep powder, but they suck on hardpack and ice. Use a minimum of weight for deep snow conditions.
For "street" tires, good luck.
For AT tires and winter treads, I had good luck all around running about 200 lbs in the bed.
As far as weight goes....
I recommend AGAINST using solid objects or objects that can freeze (IE: tubesand, cinder blocks, rocks, scrap steel, etc). Think of what can happen if you end up in a wreck. Ever see what a solid object does to the bed of one of these trucks just by stopping suddenly in the summer time? I've seen a car battery cave in a tailgate just by someone jumping on the gas on a hill and the battery taking a run on the plastic bedliner from the front of the bed to the tailgate. I'd rather not end up with my "winter weight" on my lap if I end up in a wreck.
Since I use my bed on a regular basis, most "normal" methods of adding weight do not work all that well for me. For the past several years, my solution to the problem was to use bags of lead shot (for shotgun reloading, I picked some up cheap at a garage sale). In my Ranger I had my bed covered all winter, so the bags stayed dry. At 25 lbs each, they were nice and heavy, but didn't take up much room, and didn't hardly move even in a wreck (they tend to soak up energy instead of flying around). For my fullsize trucks, I put the weight in the toolboxes to keep it dry (although my F-150 is going to end up hauling around bags of salt this winter too because I intend on having a plow on the front).
Even 4x4 trucks can benefit by having some weight in the back, my F-150 does substantially better with around 150 lbs extra in the bed during winter.
Good tires also help a lot. Even with weight in the bed of my F-150 the first winter I had it, I was using 4x4 a LOT. I couldn't even get out of my relatively flat driveway without locking the hubs in and throwing it in 4-hi. Of course, it also had cheapie street tires on it. Halfway through winter I got my hands on enough green and bought a set of aggressive AT tires and suddenly I was able to go all over the place in 2wd.
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