Years ago I wrote a list of 100 things to do before I die - simple things like "Go bungee jumping", "Dive on the Great Barrier Reef", "Eat Fugu fish", "Fly in Concorde". When i was younger I rattled through a large swathe (about 60 to 70 percent of them last time I did an informal count) of them at a terrifying pace but recently that's slowed a bit. I did a couple last year - "Hear the Pixies play 'Where is my Mind' live" required me flying to Boston for a gig (or, err, two) - and I've done one this year by getting a credit on a film, in this case "Wallace and Gromit - the Curse of the Were Rabbit".
Anyway, I'm a geek and I really like lists. Really like them. Crossing items off lists give me an almost visceral thrill. Sometimes I add items I've already completed to a list just so I can cross them off. Seriously. I'm that sad.
I figure that you'd have a Person object (which would just be the usual Ning user), a Thing object (which would have description) and then Map object which would have a Position (1-100), a Person and a Thing and whether it had been completed or not. Potentially you could then have any number of Proofs attached to a Map which might be photos or text or both.
That way someone else can view your list and then decide that they quite like your idea and steal it. And, if I'm being evil, it'd potentially be an advertising goldmine. Highly Targetted ads selling the resources needed to complete a todo? Easy demographic profiling? Ka-ching!
Shouldn't be overly hard, I just need to find the tuits.