I'm a big fan of Yahoo! Notepad, so here's an idea to improve it :-)
I always like to write down stuff because it helps me to be a bit more organized and helps me make sure that I don't forget something. So, I end up using Notepad.yahoo.com heavily. I usually add/delete lot of notes regularly - the reason is I try to keep consolidating the notes from more to less number of notes. One day, I got frustrated, and I setup my own private wiki in a hidden part of my website and I was using it well ; but I ended up accessing my mail, calendar and address in Y! but using a separate wiki instead of notepad. So, here's where my idea/feature request comes in... why not make Notepad wiki-like!
Use an extremely simple syntax like Markdown. Ability to link to other notes of mine makes it so useful!
Give a choice of having to print either the raw format or the converted HTML format. Both choices are useful depending on the situation.
This gives more incentive to use Notepad. I would definitely expect more people to use Notepad if such a neat feature was provided.
Brings in lot of geek cred!
The basic idea, however, is to bring more structure to my notes, without significant overhead.