Text editor - float above other windows
Stickies!

The OS X version of this app that's been on Macs since System 7.5 has the ability to float, and become semitransparent.



Technically it's a rich-text notetaking app rather than a plaintext editor, but it might satisfy your use case.

Another option (quite the polar opposite of Stickies) would be to use an editor within Terminal, and use TotalTerminal to display it in "visor" mode.

Finally, it looks like there was an open-source project that added these kinds of window management features to any app of your choosing called Afloat. The original project hasn't been updated in 3 years, but one of the forks might be functional on the current iteration of OS X.