Bricks

Brick is a block first appearing in the 0.28_01 update. Brick is currently only available in Creative and Alpha 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. In Alpha, brick blocks are crafted from bricks.

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.