From Ben Summers:
SpotMeta attaches Spotlight-searchable custom tags, category information and annotations to any file or folder on your computer. Design your own filing system with SpotMeta's easy-to-use editor, then use Smart Folders and Spotlight to instantly retrieve and organize your work. SpotMeta seamlessly extends Spotlight to work the way you want to work.
What's new in this version:
Remove expiry, change to open source.