Icehouse pieces is the original name for Looney Pyramids, since they were first invented for the game Icehouse. Each piece is a pyramid of one of three sizes: large piece (Queen), medium piece (Drone), and small piece (Pawn). They can be made from a variety of materials, and may be home made or purchased commercially. Commercially produced Icehouse pyramids are made from wood, plastic, paper, or stone, while homemade ones can be made from a wide variety of materials, including wood, metal, plastic, polymer clay, plaster, paper, and more. The plastic pyramids are stackable, as are some of the stone ones, and some homemade pyramids. Many games use multiple colors of pyramids.
This WWN article explains why Icehouse pyramids are hollow.
Looney Labs has published official standards for the sizes of Icehouse pyramids , allowing anyone to make compatible pieces in their own designs from a variety of materials. Pieces come in 3 standard sizes: small, medium, and large. Giant pieces are, by convention, 8 times larger than the equivalent standard size pieces. Based on the pattern observed in these three sizes, a pair of simple linear equations can be devised for the base and height of a pyramid based in its pip count, allowing the sizes to be extrapolated into 0-pip, 4-pip, 5-pip, 8-pip, and larger pieces.
- Small piece: 9/16" base, 1" high
- Medium piece: 25/32" base, 1 3/8" high
- Large piece: 1" base, 1 3/4" high
The angle at the apex of a large pyramid is approximately 31.9 degrees.
Here is Kit Cooper's old letter from 1990, describing the mathematic standards of the original pieces: 
Unofficial sizes in cm
For those of you either from Europe, or from anywhere else where you use the metric system, here are the dimensions of the standard Looney Labs Plastic Treehouse/Icehouse pyramids:
- Small piece: 1.43cm base, 2.54cm high
- Medium piece: 1.98cm base, 3.49cm high
- Large piece: 2.54cm base, 4.45cm high
Currently commercially available pieces
- Stackable plastic pyramids from Looney Labs in standard size. Available in multi-color Treehouse stashes at many game stores  or direct from Looney Labs. Single colour stashes are no longer available.
- Stone pyramids from Crystal Caste.
- Czech pyramids