I've been reading a really FUN WIP on ff.net; It's a Tenth Walker with a twist--the extra is a convicted con man who volunteers for an experiment to shave some time off his term; it lands him in Middle-earth just in time to meet up with nine strangers who take him prisoner...
"The Ex-Convict's Guide to Surviving a Medieval War (in Six Bullets or Less)" by South of the Storm I love the OC's very snarky and cynical POV, the language barrier, and seeing canon events through his eyes. His take on the various members of the Fellowship is interesting, very sharp in some ways and totally off-base in others (which I won't reveal for spoiler reasons). They've just arrived in Lothlorien; best chapter yet, IMHO. The author is already showing how being with the Fellowship has helped with the OCs character growth. The story's a blend of book and movie verse, leaning a little more on the latter with most of the plot, but including a lot of book elements as well.
Even though it's a WIP, the author is updating frequently, so I feel safe in saying it's truly in progress. If you like Quest fic, and you like the occasional well-written intruder, well, give it a shot and let me know if you like it! (And do comment at the story too!)