I've been holding off on this one for a while, because I'm not sure if it qualifies as a Fan Film or not.
Basically, back in the 1970s, there was a show called "Space: 1999," which has been much maligned, and more-or-less rightly so. It was beautiful, but simultaneously pretentious, incoherent, goofy, and suffered from an unfortunate overdose of "Like, wow, man!" It was cancelled after two seasons, and despite the occasional failed revival attemtp, there it languished in infrequent syndication purgatory.
In 1999, Johnny Byrne, script editor and occasional writer for Space: 1999 wrote a script for a short film, which was filmed at an SF convention in Europe. In essence, it functions as an expositional monolog, wrapping up the story and blatantly breaking the 'show, don't tell' rule inherent in all television and movies. It was kind of touching and nice, however, and it does give some closure to such people as still needed some closure (I'm pretty sure I fall into that category).
So is this a Fan Film or not? I'm not sure. It features a former cast member from the show reprising her role, but then so does Star Trek: Phase 2. It features a script written by a writer from the original show, but then again so does Star Trek: Phase 2. It appears to have been privately financed, and later gobbled up by the studio for the DVD collection, but I'm not entirely clear on that, and I don't know if this counts or not.
At *best* it's a semi-pro production, not an 'official' one, or at least not intended as such.
I have to say, Miss Merton is holding up really well. Here's the same thing, but fanedited into an "Episode" with fake titles and stuff. I'm including it mainly to show a fan edit of a fanfilm, which is kind of a neat concept, and also because it includes an unused alternate version of the Season 2 theme.