High Priority is an inexpensive Mac OS X 10.4 or higher menu extra whose sole purpose is to help you manage your To Dos in a simple, intuitive, and pleasant manner.

A triggers-centric tutorial on using Quicksilver. (I set up a lot of triggers when the feature first appeared, but tend not to use them much - it feels more 'natural' to me to use the QS interface, if a teensy bit slower).

"Essentially, KGTD is a single OmniOutliner document with some fiddly bits (ok, we can call them “buttons” if you are going to get all technical) that allow you to manage two views of your GTD tasks: projects and actions."