Seeking OS X app recommendation: instant text entry
You can do this without any third party software. Just use Automator to create your own Service that appends the input string to any text file.



Open Automator and select to create a new Service that receives no input in any application.

From the Library, drag the Run AppleScript action to the area on the right, and use the following script code:

on run {input, parameters}
    return text returned of (display dialog "Enter some text:" default answer "")
end run


Then, add a Run Shell Script action that receives input to stdin, and replace the default script with the following:

cat &gt;&gt; /Users/danielbeck/notes.txt


Instead of /Users/danielbeck/notes.txt, specify any path.

Here's what it looks like in Automator:


  


Save as e.g. Add Note, and assign a keyboard shortcut in System Preferences » Keyboard » Keyboard Shortcuts » Services » General » Add Note

Here's the dialog box:



You can press Cancel, Escape, or Cmd-Period to cancel.



You can even change how the notes appear in the text file through changes to the Run Shell Script action. For example, you can use the following instead, if you reconfigure the Run Shell Script action to receive input as arguments instead:

echo $( date +"%Y-%m-%d %H:%M:%S" ) "$@" &gt;&gt; /Users/danielbeck/notes.txt


It will look like this in the resulting file:

2012-01-30 12:38:56 foo
2012-01-30 12:38:58 bar