I do love my keywords in Aperture. And I've used them to keep track of items such as what plugins I use when processing a photo. In a recent article on Aperture Expert, Walter Rowe showed how he uses Aperture's custom metadata fields to track submissions to online portfolios. 

The advantage over keywords is that the custom metadata remains in Aperture. Tags in exported images don't get muddied with entries like "500px" or "Shared to G+". In the video below, I walk through how to convert a keyword into a custom metadata field. Aperture makes this quick and simple.