I have bags of dice.
Bag one has dice that roll a certain way, I have a D20 that favors low, and one that favors high. I also have a D6 that always rolls a five. The other dice are not as predictable, but still roll fairly high most of the time. I know it is because of defects, or weight balance, or other defects. I really don't care, I use these as a player, I enjoy getting a bit of an edge, and consider it like my PC using the right tool for the job.
Bag two are some of teh most consistantly random dice I have ever found. I use these when DMing an important event. I only have a d20 and a D8 that are honostly random, and make the PC's use the D20 for certain climatic events. They know something important is happening when I put the blue/gold swirled die on the table.