This paper describes a novel music playback interface, called
Musicream, which lets a user unexpectedly come across various musical
pieces similar to those liked by the user.  With most previous
``query-by-example'' interfaces used for similarity-based searching,
for the same query and music collection a user will always receive the
same list of musical pieces ranked by their similarity and
opportunities to encounter unfamiliar musical pieces in the collection
are limited.  Musicream facilitates active, flexible, and unexpected
encounters with musical pieces by providing four functions: the
music-disc streaming function which creates a flow of many
musical-piece entities (discs) from a (huge) music collection, the
similarity-based sticking function which allows a user to easily pick
out and listen to similar pieces from the flow, the meta-playlist
function which can generate a playlist of playlists (ordered lists of
pieces) while editing them with a high degree of freedom, and the
time-machine function which automatically records all Musicream
activities and allows a user to visit and retrieve a past state as if
using a time machine.  In our experiments, these functions were used
seamlessly to achieve active and creative querying and browsing of
music collections, confirming the effectiveness of Musicream.