I have Notepad2 installed in my /windows folder and I really prefer it over the standard offering since it comes with a whack of useful features (like line numbers, scheme formats, regular expression searches, etc.).  Unfortunately, it isn't obvious nor easy to default IE to use it for “View Source” operations on a web page.

So, here's a quick way of doing this.  Disclaimer:  This involves editing the registry, so be careful, don't hold me responsible, yadda, yadda, yadda.

1. Open up regedit from the Start..Run box
2. Navigate to HKLM\Software\Microsoft\Internet Explorer
3. You should see a key in there called View Source Editor;  if you don't (like me), add it.
4. Within that key, if it doesn't exist, create another key called Editor Name.
5. Modify the default value of this key to the fully-qualified-path to your editor of choice.  For me, I used “C:\windows\notepad2.exe“.
6. Close RegEdt

Changes should take effect immediately.  Enjoy!