Glass

Glass is a decorative block that Sunlight can penetrate through the pen; making it useful for buildings such as greenhouses. There are also Glass Panes. It has an almost completely transparent texture put on all six sides of the block. Glass blocks adjacent to other glass blocks are invisible when viewed through glass.

Glass will not hinder a redstone current from traveling down the side of a block diagonal to it.

Redstone Wire, Jack 'o' Lanterns, Rails, Doors, Pressure Plates, Beds, Trapdoors and Ladders cannot be placed on (or against, for trapdoors and ladders) glass. Torches and Redstone Torches can be placed on top of glass blocks, but not attached to their sides. Glass can be re-obtained by a tool with Silk Touch.

Trade
Glass blocks can also be obtained through trading with villagers.

Trivia

 * Glass blocks do not reduce the intensity of the light passing through it, aside from the normal decrease of -1. See Opacity.
 * Rain and snow cannot be seen through glass that is being broken.
 * Snow does not accumulate on glass.
 * If you try to light glass on fire with a flint and steel no flames will appear, but the flint and steel will still act as if it had been used (its durability will decrease slightly). The same thing happens with ice.
 * Placing a glass block on top of a chest does not prevent the chest from being opened.
 * Glass towers can create light underwater via sunlight.
 * Glass placed next to water running downwards will make the water run more quickly and violently; this can be useful in making traps.
 * When Glass is placed over a dropped item, the item is trapped inside the glass but can be retrieved normally.
 * Mobs (excluding Zombies, Spiders and Slimes) cannot draw line of sight through glass.
 * Narrow (1x1) towers of glass which mark Strongholds can be naturally found in worlds generated in Beta 1.9pre3. This is not a bug, but a debug feature which Jeb forgot to remove.
 * The texture for Glass does not support partial transparency as Ice does. If the Ice texture is applied to Glass then many of the blocks behind the Glass become invisible and you see The Void.
 * Hostile and passive mobs are unable to spawn on Glass, regardless of the light level.
 * Glass can be picked up using a tool with the silk touch enchantment.
 * While glass panes are more commonly used for windows, glass blocks are usually used for curved glass structures, such as domes.
 * In Survival test Glass could be broken very quickly and would drop a glass block to pick up. The only way to obtain glass blocks at that time was to place them on the map before the level was loaded in the client.
 * When riding a mine cart and a glass block exists one block above the ground only, a player can ride through the glass in a mine cart. This can be used to make a structure that you can ride into without a mob being able to enter the structure.
 * You appear to be unable to see glass through an adjacent glass block, much like ice vs. water. (Tested in SMP Creative, so this may be a bug.)