Oh, this is the best yet, category fans.... (or at least it appears to be, to my non-script readin' tech-bodging eyes....) If I understand correctly, M. at Pappmaskin has written a script that executes a full-text search of your blog for in-post keywords, and displays the results (gasp) in your blog, and on your template. This seems spiffy!!
You have to hand-code search terms into your sidebar, & because this category method is a search, you don't get the traffic benefits of tagging, but for those of you who have been "on my case" for steering my traffic the way of del.icio.us, or who feel that the method I'm using is clunky, this may be for you.
I can't claim to grok it 100%, so if you do, hit the comments & set me straight!!
Update 1/14/06: For a clear & concise how-to for this method, see Young Ladies Christian Fellowship. Thanks, Gretchen.