I hate SmartTags or things that change your content (or mine) by default in the Web browser. Adding links to my content without my control is not something that I can support. It is anti-Web, in my opinion.
And kids... guess what Trixie (and Greasemonkey, for that matter) does for you? As I argued last time round, I like the control.
So far, I've built form fillers, and I'm moving on to some "taggy" things next. Mmmm, tags. Automatic tags. On every site. Yum! (grin)
[Update 9 Jun] OK, so Trixie's now been superseded...