>>So, who are the bands over the last 15 years that only had one 
>>release but you consider it essential.

My essential one-album wonders...

IMPERIAL DRAG - s/t
CHERRY TWISTER - At Home With
THE DIGGERS - Mount Everest
BEGGARS - s/t
CHOPPER ONE - Now Playing
FRISBIE - Subersive Sounds of Love
THE ORGONE BOX - s/t
  (I know there was a set of 4-track rarities...but I don't count that :)