I saw a request on the Notepad++ forums today to add PowerShell syntax support. Notepad++ has a pretty good extension system for adding user defined languages, so I went ahead and put on together. You can grab it from the Notepad++ download area (zip download). Installation information is in the included readme file - you extract the zip, rename a file, and drop it in your %APPDATA%\Notepad++\ directory.1
I've submitted a patch to get it included in a future Notepad++ release. The patch has been accepted; it's available for download from the Notepadd++ site.
The PowerShell API is very discoverable - you can get a list of all supported PowerShell commands by running the command "get-command" with no parameters. Similarly, you can get a list of all command aliases by typing "get-alias". I also threw in all the standard WMI classes and the basic PowerShell operators and syntax based on the online documentation. You can read my walkthrough of adding a User Defined Language to Notepad++ if you'd like more info on the process involved.
I'm thinking this might be helpful for occasional quick edits. For a real PowerShell editor, I highly recommend PowerShell Analyzer.
1The only gotcha there is if you've already added a user defined language for Notepad++, in which case you'll need to merge the files.