When I have wanted to write myself an not, I have been using Stickies or writing the text into a shell window. This isn't ideal, since I normally don't keep Stickies running all of the time, and typing into a shell doesn't really work, especially if it is a multi-line note.
I just found Sidenote yesterday. It is a little program that puts a drawer on the side of your screen. When you move the mouse over to that side of the screen, the drawer opens up. Then you can type or paste your text into a note.
Sidenote can handle text with different styles, while in Stickies, a note can only have one style. You can also copy graphics, sound and video into Sidenote notes.
The one feature that I wish it had, was the ability to convert a url string into a clickable link.