I tout documentation quite a lot, but the specifics behind the actual documents have been a little fuzzy. For instance, having an internal wiki is invaluable, but it's as easy to create crap documentation just as well as good documentation (probably easier). What goes into a good document? What form should it take? There are no easy answers.
As with many things, one of the best ways to learn is to examine what other people have done, and that's where Craig Borysowich comes in. If you don't read his blog on IT Toolbox, you should. He consistently produces excellent examples of documentation with his Deliverables series. He also completed posting an example of a system blueprint. For anyone who hasn't already done something like this (like me), it's an amazing time saver, since Craig has done all the hard work.
Like I said, if you're looking for excellent examples of documentation, you owe it to yourself to check out his blog.