I found PocketInformant shortly after purchasing my PocketPC (Dell Axim), and have been very pleased with it since I got it. My favorite features are the saved views, category grouping, and drag-n-drop to change or add categories. I use categories extensivily to organize my tasks by context (e.g. at home, at computer, etc.) and being able to move stuff from one to another is very useful. Of course, it does force me to use just the one task folder, as it relies on ActiveSync and Pocket Outlook for the task data. A more comprehesive solution that could sync multiple folders of tasks, calendars, and contacts would be really useful. I currently keep a projects list by marking them as important and a perhaps list by marking tasks as low priority. This works ok, but it would be nice to have that additional information on tasks that I'm working on for sorting, etc.
Categorization is a hard problem to get right, because many people would use it to classify items on multiple axes. For example, I have a set of categories that are only used for next actions I'm working on, but I also have a set of categories that reflect areas of focus in my life, so that I can categorize "Plan dinner out with my wife" as something to do at the computer ("@ Computer" category) and also as part of my work to be a good husband ("Husband" category). I could also add another classification with categories for projects ("Fall in love all over again" category/project). However, in both Outlook and PocketInformant, if I just want to look at my next actions (organized by category/context), I will also see the other two categories I've placed this task in. Ideally it would be easy to add a new classification system (Categories 2) that allows separate entry, filtering, etc. to make it easier to organize along the important axes. Obviously, things like categories (labels in GMail) is a good start. Now, how can we step back a level to make it easier to organize categories into useful and usable groups, without avoiding the dreaded hierarchy? Sounds like a fun problem to solve.