Ostinato - is a repeated rhythmic pattern. Here is an example of the percussion group Synergy performing on mallets. The bigger mallets are playing the rhythmic ostinato.
Polyrhythm- is two or more different rhythms playing at the same time.
Free Rhythm - is music that flow through time without a sense of metre or strong and weak beats. In this example this Bouzouki player performs his improvised performance using free rhythm.