First, the really bad:
Calcium sand: It's coarse, it's not digestible, it can cause your dragon to over dose on calcium. Not to mention T-rex makes it in all sorts of strange colors, so not only do you have purple sand, you have a purple beardie too!
Lizard litter or whatever it is: That stuff that is clearly indigestable dispite claims from the company that it is. I think ESU makes it? Don't get it confused with the ESU Jungle lizard litter blend, which is basically a combination of dirt, sand, and some other stuff (more on that later).
repti bark or similar products: If your beardie ingests this it's most likely going to kill it. Enough said.
Pebbles or aquarium rocks: This is pretty much too obvious.
Now, for the good ones:
Play sand (or zoo med's repti sand, or similar non-calcium and non-silica based sands): Most people have good luck with this, although there is a risk of impaction. I think most impactions occur because either the dragon is fed on the sand, or it licks it too much. Sand can more easily impact babies, though many people still use it without problems (which is not recomended at all).
Repticarpet or similar indoor/outdoor carpets: This stuff seems good, but what most people don't realize (I didn't), is that though a dragon can't get impacted on carpet, you have to bleach the carpet every time the reptile soils it or else your dragon could fall victim to bacteria.
Bed-a-beast: Though it's digestable (so they say), you have to soak it in water to get it to expand to full size, and then you won't be able to get enough water out of it to maintain the low humidity that bearded dragons require.
ESU Jungle blend lizard litter or other soil based substrates: There hasn't been much research done on these yet, but I am led to believe that it would be easier to pass than sand. If so, the only problems are that it's dark in color, requiring more lights on the tank to get the kind of intensity beardies desire. It will also most likely hold moisture for a while too, but I think the heat lamp would take care of most of it. I would think these substrates would do better in combination with another substrate. A tank filled with sand on one side and soil on the other seems like a good idea. This substrate is also nice if you want to put plants in the enclosure, as you can plant them directly into it.
paper towels, newspaper, etc.: These work extremely well for baby dragons, the only downfall is that it doesn't look too great. Also, it doesn't work as well for adult dragons because of the lack of traction. This is the ideal quarantine substrate.
Okay, I know I'm forgetting some stuff, so if you come up with something I didn't talk about, give us your opinions on it. Also, if you disagree with something I have here, correct it.




God, don't I wish mine would poop on a little square like that. Mine like the hammocks which they bask on so they know I'll get right in to clean them up
spoiled rotten.