R.I.P. - Remove It Permanently. Overlaps with AdBlock in functionality but does things AdBlock can't do in removing HTML elements that clutter up the page. Killer feature is it shows you what will be removed before you do it.

Tab Mix Plus - best of the tab pref extensions, imo.

Google Suggest - uses Google's predictive typing in the standard search box

This customization of ForecastFox.

Lots more but those are the most important ones to me. And I really like the Noia Extreme theme.

I'll check out Target Alert again. I had it installed and I liked the concept but not that it messed up the HTML formatting.