It depends on the gaming milieu.
For an FtF game, physical dice. For my 4e rules solo campaign, I use the dice that came in the box set (so that was my poll choice). There's something psychologically satisfying that comes with the tactile experience of rolling
real dice. Rolling a d12 just
feels better than selecting d12 from a drop-down menu or whatever. I think that using physical dice also gives the roller more of an illusion of control than virtual "dice" do. It's a lot harder to blow on, or kiss, or shake digital dice extra long before rolling.
For PbP where transparency is important (or trust is lacking), an online dice roller that displays results (publicly or privately) is a must.
For the Ref on the go, there are a couple of free dice roller apps that you can install on your smart phone. I use one called
RPG Dice Roller that I would recommend. You can choose d2-d100 and include modifiers (or not). It's got roll history and save functions, and customs rolls. It's also got something called "fate" that randomly generates a +1 or -1, but I'm not sure what that would be used for.
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