To tag a file - just drag it onto the Tagit application in the dock.
You can also assign ratings. Tagit lets you drag multiple documents to it at once. With multiple documents, Tagit will add tags to all the documents, leaving individual tags on specific documents alone. Which is just what you expect and need. Your most recently set tags are easily added with a single click.
In order to search for documents with a specific tag or rating, simply click on Tagit in the dock. A handy search window helps you pick tags and or a minimum rating to search for. After you use the search feature a few times you can see how easy it is to search for tagged files without even starting Tagit - you simply type a search like 'tag:splash' into the Spotlight search window on your computer.
Tagit is a free application - It will always be free. Tagit uses an open source document management system known as OpenMeta. With OpenMeta, your tagging and rating efforts are backed up as you back up your files. OpenMeta is quickly becoming the standard for document management on OS X.
Mac OS X 10.5.5 or higher.