The best way to find out about these toys is to try them out; they're
all small demos that have few/no dependencies beyond the clutter
libraries. The following is a summary of each;

arc-clock     - A small clock demo (using the cogl path API)
circles       - A small spinning circles demo (using the cogl path API)
nyancat       - A small nyancat made of clutter
courasel      - A carousel-style menu demo
foofone       - A mock-up mobile phone interface
gps-globe     - The start of an app to draw a globe with your GPS pos
object-store  - A ClutterModel subclass that proxies GObjects
opt           - A slide-show application
odo           - A mesh deformation demo set
pinpoint      - A minimalistic presentation tool
pong          - An unfinished pong game
ripples       - A small 'ripples' effect demo (using the cogl vector-drawing
                API)
script-viewer - A ClutterScript viewer

Bitrotted toys that do not work with Clutter 1.x API yet have been placed in the attic/

aaina         - A Flickr picture browser
astro-desktop - A mock-up desktop/palmtop application environment
fluttr        - Another Flickr picture browser
gcr           - A Gegl-based clutter video recorder
mallums-magic-browser - A webkit-clutter based web browser
sqlite-model  - an sqlite-backed ClutterModel implementation
table         - A multi-touch table style picture/video viewer (not actually
                multi-touch)
widgets       - A texture-reflecting actor demo
woohaa        - A video browser/player
youhaa        - A (currently broken) YouTube video browser