One of the most important project-management assets you build up during your project is documentation of all kinds of things -- how you broke down estimates, how much work you completed each work-period, what kind of risks actualized, etc. etc. and this is all useful because it will help you in future projects that are similar, whether greatly or slightly similar.
As mentioned in previous questions, I work primarily on one-man software projects using agile methodologies. I don't have the benefit of a PMIS, a "Project Management Information System" that consolidates, archives, and indexes my documentation so I can search through past documents quickly; nor do I have the cash reserves to purchase or implement one.
How, then, do I usefully archive all past project documents that may be valuable? I'm trying to keep all my documentation online as Google Docs, breaking down projects by folder, and having an "archive" folder of completed projects.