I need a better way of organizing my notes. I have always been in the habit of just using a single text file, where I track and note various things. But, that file is now very long. It works well for me, but I think I duplicate some things, and some things I don't need anymore, or do need but don't need them in the file.

How do others organize these things? There is probably some OS X app which could better organize these things, and I just haven't seen it. It's probably noted in the text file to look for something.