I'm building up towards a piece on iPhone and iPad apps for looking at art - in the meantime, have a look at the newish, heavily social/interactive artfinder site (and Jonathan Jones' review in the Guardian). Jones categorised the artfinder site as an 'online museum', a move the site's creators responded to in a blog post
There’s one thing we wanted to correct though and that is that we’re not an online museum. We’re a website. Museums are special, magical places and not all online things have to be an analogue of a real world thing. We’re proud to be a website, it’s what we always set out to be. There are points when we’ll do museum-like things. We’ll curate, juxtapose, hopefully inform, delight and convene. But these are things that websites can do too. What we hope to be is a website where related works from disparate museums can be brought together so you can find more art and artists that you love. We could spend all our time trying to replicate every minute facet of the museum experience in a virtual space, but that’s not the intention of Artfinder. The intention is to be a new way to find and collate and share the art you love and we feel the best vehicle for that is a website. There are other parts of the service that we’re working on at the moment where the best vehicle may either be an app on a smartphone, or an app on a tablet device. These won’t be analogues of museums either, they’ll be their own things, but essentially, they’ll be apps and we’ll be proud of that fact as well.
Also recentish - the Guardian's book site for children (which looks remarkably like its books section for adults, but which is aimed at 7 year-olds up to teenagers, and encourages them to write for and interact on the site). If you prefer your reviews by adults, the New York Times archive of children's book reviews is quite something.