Most important documents
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If you are about to hand over a fairly complex software system to someone for them to maintain and develop (co-worker, system integrator or customer), which type of documentation do you think is the most important? (in addition to well documented source code)

I'm looking for what's actually **useful**. There are lots and lots of documents I can think of but documentation is allways hard to maintain and tends to get low prio, so I'm really after the golden nuggets.