VoodooPad is one of these tools that have become indispensable for me. In short, it’s a Wiki for your desktop (well, Mac OS X desktop, to be correct). I use it for everything, outlining, keeping my action lists, organizing my thoughts, keeping reference of information I need every day, like code snippets, small FAQs and so on.
While v2.0 was a great tool, v2.1 took it to the next level and introduces support for Spotlight, Categories (a.k.a. tags), inter-document linking and lots of small things. One of my favourite features is still the export to iPod. An awesome tool, be sure to check it out.