OmniOutliner is a wonderful piece of utility software that I use every day for tracking small To Do lists and Expenses.
If you are looking for something to handle larger amounts of information then take a look at NoteTaker from AquaMinds (http://www.aquaminds.com).
I used to carry a log book around all day, for taking notes, but now I've gone completely over to NoteTaker on my PowerBook.
The two packages complement each other very well. OmniOutliner excels at smaller dynamically defined list structures and gives you support for dates and column totals. Meanwhile, NoteTaker handles larger document-sized outlines, allowing you to clip graphics and other documents and insert them into you note books, which you can then publish as a website directly from the tool. NoteTaker also supports To Do lists but they aren't half as awesome as OmniOutliner's.