The Antoninos

On the first three albums (von, von brigði, ágætis byrjun), Jónsi (o vocalista) sang most songs in icelandic but two of them (von and olsen olsen) were sung in 'hopelandic'.

Hopelandic (vonlenska in icelandic) is the 'invented language' in which Jónsi sings before lyrics are written to the vocals.
It's of course not an actual language, rather a form of gibberish vocals that fits to the music and acts as another instrument.

Hopelandic got its name from first song which Jónsi sang it on, hope (von).