The block on the right of this image:
http://www.minecraftwiki.net/wiki/File:Newstoneslab.png

Is gone and replaced by Quartz Slab as of the newest weekly snapshot (13w02a).

The block is a favorite of many players, including several on my own server, as seen in this reddit post: http://www.reddit.com/r/Minecraft/comments/16bp7i/mojang_finally_removed_this_in_13w02a_farewell_my/

Please bring it back!