I believe in Jesus Christ, my Savior. If you do too, and aren't scared to admit it, then copy and paste this into your signature.
I read that a while back and was surprised to find it out. A casual goodbye would be "ja ne" or "ja matte". "Sayonara" really does translate as "farewell".
Another word that's freely slung around by fanfic writers is "aishiteru" or "I love you". But to the Japanese, that was a very profound thing to say and would only be said to the person you were in love with and only in private. Saying it was probably akin to asking the person to marry you.