http://www.editpadpro.com/editpadlite.html

This one is called EditPad Lite. It's my new favorite. Go to the page to find info about it. It has a tab feature.

http://www.liquidninja.com/metapad/

This one is called MetaPad. It was my favorite until I found EditPad Lite. It doesn't have tabs.

http://www.notetab.com/

This one is called NoteTab. It's okay, but I prefer the other two.

http://www.tranglos.com/free/keynote.html

This one is called KeyNote. It's like EditPad Lite, but has many more features. It uses .knt files instead of .txt files, though.