The developers claim Pluto started as a joke although I don't really see the punchline - Pluto could potentially be a very useful tool.
Pluto allows you to store text-snippets, notes and outlines and access them with ease from your menu bar. So, if you have a sudden blog idea, or remember something you need to do, just add it to Pluto. It goes beyond just a note taking tool however because it integrates with iCal meaning you can export your notes into iCal.In addition you can export data from Pluto into other applications for further editing as well as import data too.
However, although Pluto is easy to access at the top of your menu bar (represented by a small tick) it doesn't really make adding notes as easy as it could do. You always have to click "Add" rather than just clicking directly on the note taking panel and enter a note. In addition, you can't seem to highlight more important notes in different colors. It could also do with some way of categorizing your notes when they are entered.
Pluto could be a very useful tool although the developers have a bit of work to do before I'll be relying on it to record my to do list.