Since people seemed quite taken with the first set here are five more, probably better known than the first set.
Living History by LizMarcs
Rating: PG, Worksafe
Some people have come from a place far, far away and a time far, far ahead to ask for help on an urgent matter. The time they were aiming for? February 2008. The place? Moscow. The problem? Somehow they've ended up in Cleveland, September 2003. And that’s just the beginning of the trouble; they seem to be descendants of the Scoobies and are determined not to change the past... but they haven't reckoned with Xander. Superbly written with a fascinating plot, some of the most beautifully crafted OCs I've ever seen and a wonderful portrayal of Xander and Faith. Very highly recommended!
I Am What I Am by MMcGregor
Rating: M, Worksafe (except you'll be crying for reasons you can't explain)
This is a highly original Halloween fic where Xander dresses as himself, and thereby ends up gaining all his future self's entire lifetime of knowledge of what is to come. It's one of the most outstanding stories that I've ever come across, both in theme and writing.
I didn't include this one in the first set because I assumed everyone would have read it. If I'm wrong and you haven't then RUN don't walk, find a computer and READ IT. You will not regret it. For those who have read it already, perhaps you aren't aware that the author also created a
parallel posting where the text is interspersed with writer's commentary.
Just One Night by Vogonguard
Vogonguard is probably better known for his story "The Vor Slayer", a crossover with the Miles Vorkosigan saga. This one is a relatively short story set quite literally immediately after the series end when they're all standing around the bus looking back. Someone makes a wish and... they end up back in S1 Sunnydale for one night. What do they tell themselves? Everything or nothing?
Compelled to Play Again by Hotpoint
Consisting of five stories, the first being linked here, this series is set in a seriously AU world where Illyria goes back in time (bringing Wesley with her) after the debacle at the end of Angel S5. Arriving just as Buffy and the potentials were about to confront Caleb for the first time, she tips the balance completely onto the side of the good guys. Well, at least for a while.
Although it is, at times, a little Mary Sue this is a well written (and massive - did I mention *massive*?) look at what happens when the good guys seriously kick ass. Think Slayers with machine guns and military training and cross universe invasions. It has a great dry humour and an abundance of lines that will have you ROFLing.
Warning: Unfortunately the final story is WiP and has not been updated since 2009.
Strangely Literal by Snag
Rating: PG, Worksafe
This last one is a bit of a reach, included because it uses time travel and it was either this or the Journeyverse. And that's just far too large to rec.
The story is a xover with the Terminator 'verse where Cameron is sent back for a preliminary mission prior to tracking down John Connor. Landing in our favourite town, she arrives between the events of What's My Line and Ted and things start changing fairly rapidly when Xander is assigned to show her around her new school. It's very well written, with solid characterisation and a fascinating glimpse of how Cameron might have been affected by having people around her who weren't put off by her nature and insisted on being friends.
Warning: Unfortunately this is a WiP and has not been updated since 2010. The 24 chapters written are well worth reading but it does leave you looking for more.