This doesn't exactly qualify as fanfic, but it is worth recommending.
Apparently "Superman and the Secret Planet" was an unfilmed script written for George Reeves' Superman. Randy Garrett turned it into a comic book, available
here. He did a brilliant job of capturing the actors' expressions, and the verbiage (I'm not sure how much was taken straight from the script and how much he adapted) was very true to the original series; it was easy to imagine the actors saying the lines. If you liked the Reeves' series, you'll want to read this story.
Joy,
Lynn
