Documentation is one of those things of which you don't appreciate the value until you absolutely need the document itself. I think of it as a type of insurance to web projects; it's the type of insurance you know you'll need someday but you don't necessarily want to invest in up front. Documentation is a form of risk management.
Last October I gave a talk at EngineSummit discussing documenting your ExpressionEngine projects. I’ve written an article-version of the presentation for EE Insider. Head over and check it out if you missed the talk. I think this is really important stuff if you aren’t doing it. I’ve included the slides below as well.
Read The Dreaded Documentation on EE Insider