A 'monster collector' RPG, very similar to Pokemon (I love the Pokemon games, and was trying to attempt to emulate them with this... I make no claims of being original).
It's set in a post-apocalyptic era, where people live in a VR world for various reasons! But this world is overrun with rogue AI 'creatures' called 'Beasts', which can't be destroyed but which can be caught and used to destroy eachother... Or something.
Currently, there is an Alpha of an old version playable, but there's not much to it and it isn't represantative of the final version since I'm going to make it from scratch in AS3 at some point.
The NEW version of Beast Signer will be a sort of persistant single player MMO, with new content added on a regular basis. It'll still have a set plot with a beginning and end, but it'll unravel over the course of many months rather than all at once.
Hopefully community involvement will be a big factor too. Though you'd be unable to interact with other players directly in the game due to technical restrictions, you may be able to do things like set up a shop on your userpage to sell in-game items for in-game money, and there'd probably be 'Design a Beast!' contests every so often where the players would vote on submitted designs and the winning one would be added to the game.
It will most likely require a monthly fee to access ALL of the content (that is, there'd be 'Free Accounts' and 'Premium Accounts'), just because it'll be unique to this site and not hosted anywhere else, so I couldn't get a sponsorship. I've yet to plan prices, but worry not; it won't be expensive.