Text Tools

jEdit [My Pick!] for Mac OS, Linux, Windows
jEdit is a mature programmer's text editor with hundreds (counting the time developing plugins) of person-years of development behind it.
http://www.jedit.org/

jEdit QuickNotepad Plugin [My Pick!]
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jEdit BufferTabs Plugin [My Pick!]
This plugin adds a set of tabbed-pane style tabs to the jEdit text area. The tabs show all buffers currently open in jEdit, so clicking on a tab will easily switch to that buffer. The tabs can be placed on any of the four sides of the text area.
http://plugins.jedit.org/plugins/?BufferTabs

jEdit JDiffPlugin Plugin [My Pick!]
JDiffPlugin is a visual diff utility for jEdit that shows and highlights side-by-side differences between two files.
http://plugins.jedit.org/plugins/?JDiffPlugin