Cas
Cas is a nickname for Castiel, first used by Dean in 4.04 Metamorphosis and subsequently used by Sam, Anna and other characters as well.
The shows' writers spell the nickname "Cass" (Source). In fandom, and particularly in Fanfiction it is almost exclusively spelled "Cas".
On the Season 4 DVD the english subtitles call him "Cass" and other times it's "Cas".
One theory as to the spelling "Cass" is that the writers based Castiel on the angel Cassiel, best known in popular culture from the movies Wings of Desire and the American remake City Of Angels.
Before its use in canon there were relatively few instances of the use of this diminutive, and when it did appear it was generally pilloried, subsequently its use became widespread. The spelling used in fandom and fanfic is overwhelmingly Cas.
