I was thinking the other day, trackables are unique to Geocaching.com, there isn't an alternative scheme on any other site, right?

Opencaching (The proper one!) has GeoKrety.
http://geokrety.org/index.php?lang=en_EN.UTF-8

GeoKrety (gr. geo = ‘earth’, krety pol. = ‘moles’) is a service similar to TravelBug(TM) or GeoLutins, aiming at tracking objects you leave in geocache containers. People move these registered objects (called GeoKrety) from cache to cache and register each move with this service. The trip of each GeoKret is then presented on a map.

The service is absolutely free (contrary to TravelBug) and easy to use. A GeoKret can be anything, but in most cases it is a small puppet.

Sometimes found in Groundspeak caches (usually in Europe) and can be marked in OpenCaches as being in GS caches.

You don't need to register to log a Retrieve/Drop if you are not an OpenCacher.
(Unlike GS where you have to register!)

Not too sure about the 'Easy to use' as the site seems to be down on a regular basis...