The Guten Tag fad, first started by the user Ejbloo, appeared on YTMND on March 6th, 2009. It is based on the Raisin Brahms fake cereal ad.
The fad is based of a public service announcement from The Arts Council. It depicts a child asking what's for breakfast, then Johannes Brahms smashes through a wall shouting "Guten Tag" (German for “Good Day!”). Although the PSA is intended to encourage art literacy in children, the creators posed it as an ad for Raisin Brahms cereal.
Although it doesn't appear in the original site, the child promptly replies "It's good!" (http://itsgoood.ytmnd.com). The father grows concerned with both Bobby and Suzie's fake Brahms beards, to which the composer replies, "Don't worry! That's just the power of the arts!"
The first parody site is simply the original fad posted backwards. What makes it funny is the look of shock on the father's face as he sees Brahms (apparently) leaving.
The second parody site is based off http://pi.ytmnd.com, the pi song, but includes Brahms and the kids in the commercial singing throughout the song.
The fourth site is based off "lol, toolchest" but instead depicts Brahms riding down the stairs.