I have a need for a program like this from time to time. I've been too lazy to write one, but your request made me finally go ahead and do it.
How about this?
It's pretty simple to use:
1. Open your input file and each line is displayed in the top list.
2. Specify your output filenames in the bottom list. You can have up to 36 output files, corresponding to tags "0" to "9" and "A" to "Z".
Files (and the necessary folders) are created if they don't exist. You can choose to overwrite or append to existing files.
3. Tag each line of the input file for the desired output file by pressing the appropriate key.
The highlight automatically moves down the list as each line is tagged. You can use the Shift or Ctrl key to tag multiple lines.
If you enter an incorrect tag, just select the line (or lines) and enter the new tag.
4. When you're done tagging input lines, click the "Write Files" button and your tagged lines will be written to the specified output files.
I've got the basic application written, but I want to add the ability to save output filenames,
so they don't have to be re-entered (if you use the same files each time).
If this would be useful, I would be glad to contribute it when I'm done - let me know.