FieldWorks Data Notebook
In the FieldWorks suite of applications, Data Notebook was the first major product released, and it's probably the most polished. DN and FLEx have a similar look and feel, but DN is simpler (fewer "moving parts").
As you experience various cultural events, you may want to jot down some notes on paper, but even so you should type up the event as soon as possible so that when you or someone else reads it much later, it will make sense. You should probably also mark the event write-up in DN with metadata and all relevant OCM codes before considering the write-up to be complete.
The main advantage to using DN (as opposed to, say, a word processor) is that you can use searching/filtering to dig through a large amount of data looking for patterns and connections. One of the keys for doing this well is to mark each event with all of the appropriate OCM codes.
Unless you are serious about anthropology and intend to type up a large number of events, you may find DN to be more than you need. A word processor may be more convenient, though you'll need discipline to track metadata in a consistent way.
A detailed review can be found in the August 2008 newsletter of the Society for Applied Anthropology (p.19). This review covers Data Notebook 2.8, but very few changes have been made in the tool since then.
Important Note: According to the Fieldworks team, 2.14.5 is the last release of Data Notebook as a standalone application, as it has been integrated into Fieldworks Language Explorer (FLEx) for Fieldworks 7.