Collections of essays have been around since Montaigne --- my copy of Orwell's is practically falling apart from repeated use. Collecting blog postings is a newer literary form, but as Joel Spolsky, Eric Sink, and others have shown, it can be equally worthy.
A new entry in this field is The Open Laboratory: The Best Writing on Science Blogs 2006, which was put together in just a few days, and is available through Lulu.com. The essays are all still available online, but I've ordered hardcover --- I still find that makes folding down corners easier ;-). And if this topic interests you, check out the North Carolina Science Blogging Conference on January 20.