Bricks

Brick is a block first appearing in the 0.28_01 update. Brick is currently only available in Creative and is used mainly for brick houses and indoors (e.g. chimneys). In Survival brick can be mined for a brick block back. (You can only get brick by pre-placing the block in the map with Creative mode or another form of map editing) In Indev and Infdev brick requires a Pickaxe to bust and yields a brick block when busted.

Notch has said that there may be more kinds of brick blocks in different colors in the future.

Spawning Pyramids
For a brief time, huge brick pyramids were a rare structure that could spawn in a old Infdev version. See this video, this video and the end of this video to see the pyramids and exploration of them. The pyramids were completely solid; they had no inside rooms. A strange quality about these pyramids is that if one dug under it, a layer of dirt and stone extended under the pyramid. This suggests it spawned right on top of the terrain instead of being a complete pyramid with the bottom parts underground.

It is unknown if pyramids can still be found in present Infdev versions.