I've really no need to point at The Diary of Samuel Pepys as every bugger already has, but suffice to say, it should win numerous interactive BAFTAs and the like, and of course won't.
I showed my Dad over xmas. He's a retired history teacher and said he wished he'd had that kind of thing when he was teaching (I've written a bit about history and blogs before). The location-based information is already fascinating e.g. observe how our knowledge of Westminster Hall is building up. Link this to wireless location-based services on your phone/PDA/whatever and we'd have live layering of history going on ...
What's also particularly striking/pleasing is the sheer quality of the annotations being added to the diary, even at this early stage. This is a great example of how to build collaborative knowledge-construction spaces. I'm increasingly pointing lots of people at it.
I often use a tenuous analogy (as is my wont) comparing the design of interactive collaborative spaces (messageboards, commentable-content etc.) to the design of post-war public housing projects i.e. build them with quality, care and attention to detail, and give people a reason to maintain them/contribute with real public responsibility and they'll work cf. the Barbican, perhaps, and sadly few others in the UK - with caveats about the Barbican in the last paragraphs of my previous post on the place.
Or build them little attention to detail or sense of quality and care, and don't support and replenish them, or think about the space around them, and you therefore don't give people a reason to be constructive. Look to most British post-war tower blocks of flats e.g. Manchester's Hulme crescents, Sheffield's Kelvin estate (documented brilliantly by Pulp) etc.
So, by providing constructive space with specific use, and by placing an emphasis on quality, fit-to-purpose plus adaptability, public responsibility, and local context, one induces constructive input and interaction. At BBCi Music, we built the review comments system with that in mind, and although the system isn't fully automated yet, the editors haven't actually had to edit a thing - the comments are generally informative, entertaining, and enthusiastic. Likewise most blogs, I guess. And certainly Phil Gyford's Pepys' Diary, which balances quality and specificity, using 'local' resources (users, streetmap, photosites etc.) with approachability, adaptation, and a responsibility to add to the sum of human knowledge, thereby inducing a similar response from its users. Love it.