I've been using
NotePad2 for a while now. It's a great replacement for NotePad, the best feature being syntax coloring. I've made it the
default View Source editor for Internet Explorer, and changing the default scheme to Web Source Code, it has been extremely useful. But lately NotePad2 became a pain in the neck, especially when editing large files. It hangs most of the time (especially when you view source a page with a large viewstate, you end killing the NotePad2 process 99% of the time), actually NotePad performs better with large files.
So I decided to look for an alternative, and the best one I found was
NotePad++. It looks pretty cool, has a lot of features, more than NotePad2, starts as fast as NotePad and NotePad2, and its free. The installer gives you the option of making it the default view source editor, so no need for registry editing.
I will try it for a while, and see how it goes.