What is this block in Minecraft's terrain.png?

Solution 1:

Command Block

That block is the Command Block, commonly used for server owners with server commands. It is not possible to break it unless you are in creative mode. It can't be normally crafted. Currently, it is not available in the creative mode inventory either.

Item Frame

The other block is an Item Frame. The Item Frame is craftable with 8 sticks and a leather in the middle. This is what the Item Frame looks like.

This is how you make it.

[Images from Minecraft Wiki and Minecraft101 (dot) net]

Solution 2:

The left one is the Command Block, which was already mentioned by @Bailey Rowland.

Just writing this for completeness, the right one is the Item Frame. The border of the item frame is the texture of birch wood plank, and the back uses that texture.

In fact, on the Minecraft Wiki there is an image indicating what terrain.png represents:

Here you can easily identify them.

Just FYI, the format of texture pack was changed since Minecraft 1.5, so the information is not strictly applicable for Minecraft 1.5 or above.