I was introduced to IconBuffet (fyi, personal referral link, no money just "game points") [1], which might be described as Pokécon or World of Iconcraft. It takes a large and growing collection of "free" [2] icons from a handful of talented Icon "Chefs" and creates an artificial scarcity/value for the icon sets by wrapping them in a simple economy game, associating the sets with time and effort and giving a bit of extra interest to what might otherwise be glossed over as "yet another collection of 'free' icons". The game mechanics encourages active community members that they've "got to collect them all".

The game design uses three pseudo-currencies and award 'badges' that will be familiar from a number of other websites and several MMOs. [3] The currencies are Tokens, Stamps, and Points:

The "postage" required to send a set that you have in your collection to someone else's collection. You gain stamps whenever you accept the delivery of a set that someone else has sent you. (This is done at an asymmetrical exchange rate to encourage sending but not spamming: sending requires a range of stamps from 1-10 depending on the quality/style/rarity of the set you are sending, whereas redeeming a delivery yields 5 points regardless.)
Tokens are paid for each delivery you redeem and are equal to the "postage" of the delivery you receive. Thus sets are somewhat "payment on delivery". Tokens are generated at a steady rate with respect to time, beginning at 10 tokens per month for a new free membership.
Points are the repetition/friendliness/community score and are received for each successful delivery you make to someone else, as well as through other means such as successful referrals. Points are not redeemed, but reaching certain scores generate bonus prizes (such as additional tokens per month).

It's a very unique system that seems to have a fairly vibrant community and a unique gift economy. It certainly makes a very simple observation area for those interested in game-based economics observation.

[1]Via Brass Goggles: IconBuffet's Steampunk Icons
[2]As in beer, but sometimes as a bonus as in speech (ie, vector), with a fairly open icon license agreement. It would be awesome if they used a common CC license, but their license reads similar to CC by and their differences seem reasonable for what they seek to accomplish.
[3]Interestingly I believe I've seen IconBuffet referenced as one of the first innovators of badges, at the very least for community-based websites if not for MMOs or games in general.