Hell's Katz Squad

Personal favorites:
http://bigtimestride.ytmnd.com/
http://ninjaworksit.ytmnd.com/
http://mathpwned.ytmnd.com/
http://whatishl.ytmnd.com/ (related to the "What is Love" fad)
http://threesecondsmores.ytmnd.com
http://pi.ytmnd.com/
http://conanomg.ytmnd.com/

"Famous" YTMNDs
http://picard.ytmnd.com/
http://blueballfixed.ytmnd.com/
http://tcruiseko.ytmnd.com/
http://ckjcwf.ytmnd.com/
http://jonesrock.ytmnd.com/
http://conangator.ytmnd.com/ - then a bunch of spinoffs: http://conanmakespopcorn.ytmnd.com/ , http://captainconan.ytmnd.com/ , http://conansummons.ytmnd.com/
http://captainconan.ytmnd.com/

Note: for some of them, you need to refresh after the gif / music loads, to get proper sync.

Yeah... I've got way too much time on my hands.