If you're using Outlook, I recommend Chapura's KeySuite
Keysuite (from www.chapura.com - same folks that make PocketMirror sync software) is a killer app for GTD, IMHO.
It allows you to have as many categories as you want and synchs directly with Outlook and the Palm (for notes, tasks, calendar, and contacts). You can link contacts to tasks and notes, and it works just like the built-in Palm apps.
I switched from Bonsai to this because it integrates so perfectly with Outlook and the GTD Outlook add-in.
"The world is round and the place which may seem like the end may also be only the beginning."
-- Ivy Baker Priest