The 'easiest' trackable items are Travel Bugs: a simple aluminium plate with a tracking number. You affix the plate to something that you want to 'release', and it's off. We've seen metal figurines, but also PCBs from a deceased harddisk, and pretty much everything in between.
We gifted a TB to both babarage and gertvr and xaviar_nl -- it's simply fun to see where your bug will end up through its travels. (And, as my father says: "Small gifts keep the friendship alive". It's fun to give something like that away.)
The other type of trackable item are the Geocoins. Basically, these are 'coins' with their own tracking number. There are national geocoins, but there are also a lot of 'personal geocoins': coins commissioned by a single caching team. This thread on the geocaching.com forums contains a lot of high-resolution photos of some of the coins out there. Some of these are kind of 'blah', but others are amazingly detailed and beautiful.
A few weeks ago, I saw a pre-announcement for the 2007 Compass Rose geocoin. I was curious about the whole geocoin phenomenon, and when I saw this design, I decided to order two.
This is the front:
and this picture also has a shot of the back:
We found them on our doormat on monday evening. They are surprisingly large (about as big as a Compact Flash card), and really beautiful. We'll be activating them soon, and carry them along with us -- if you meet us in the field, you can log them as 'discovered'.
I don't think we'll be collecting these things (there's the cost factor, and it is more 'stuff'), but I don't regret getting these.