Ghost Action [via] (shareware) is more of a GTD-oriented to-do list, sorting your items according to projects and contexts; a rather no-frills approach, with iCal synchronization, that wouldn’t do it for me (I have no use for a pure to-do-list manager) but seems pretty functional.

SpotMeta [via] (open-source) lets you define your own Finder metadata sets, tag any file with a right-click, and create smart folders based on that custom data. Much more structured than the usual web 2.0 free-form tagging, which can be an advantage or a flaw depending how you like to work — I like the concept, but wouldn’t really want to use it.

Punakea [via] (freeware) is the free-form, del.icio.us-like alternative. You drop a file onto an auto-hiding sidebar to tag it, browse your tags with a “related” system, and create smart folders simply by dragging a tag onto the desktop; the feature I like most, the one that integrates best with my workflow, is that you can set Punakea up to automatically move any dropped file into a special folder where all your stuff will reside, making it the one-stop dropzone I always wanted to have.

I’d be using this at once if it weren’t for two problems: first, I don’t really, fully trust OS X not to lose Spotlight metadata at some point and make the whole mess unsortable; second, Spotlight is dog slow on my machine (I think there are way too many small files on my hard drive), so my good old manual folder structure is much more usable for now.

I recommend you do give it a try, though; it’s really well thought-out — even if to use it as a simple bookmark manager, a kind of local del.icio.us.