This is a freeware / open source app, and even though it's been laughed at ( "... the open source fanatics even have to make their own version of Notepad ..." ) it's extremely useful. Particularly for web developers and designers, although it seems like "tabs" are catching on, so it might be useful outside of these disciplines.
But for web people, Notepad++ is great because it has syntax highlighting, for html, css, xml, asp, php, and other languages. And it takes over Notepad's associations, so if you hit "view source" from Internet Explorer, you get syntax highlighting for the page's markup, and text folding. ( That's the ability to fold up a div, table, or whatever, to get at what's under it without having to scroll. )
SourceForge.net: Notepad++
Personally, I think syntax highlighting is the best web dev innovation since caffeine.