TOCC is a Tool for Obsessive Compulsive Classifiers. It implements a file system that uses tags instead of folders, allowing you to classify files however you want. For example, you could tag photos by location, people, and technique (e.g. black & white, HDR). Finding files is as simple as running a query like "give me all files tagged photo and my-love".
TagSpaces lets you organize, tag, and browse your files, photos, and documents on many platforms. It lets you create and manage notes and Web snippets (like in Evernote, but completely offline) and create a personal Wiki system. It features integrated MHTML, PDF, ODF, and image viewers, a Markdown viewer and editor, an advanced WYSIWYG HTML5 editor, file tagging based on filenames (extremely portable), a simple source code viewer and editor for the most common programming languages, and basic file and directory management.