Cas

Misha Collins plays Cas or is it Cass?

Cas is a nickname for Castiel, first used by Dean in 4.04 Metamorphosis and subsequently used by Sam and Anna as well. Few instances of its use in fandom before this, subsequently its use became widepread.

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.