My favourite BtVS-inspired fanfic is probably City Girls, which is about Buffy and Dawn post-Chosen. (I.e. the finale; for the record, it's canon - thanks to a season 5 AtS remark - that they end up in Rome.) You can find it here:

http://www.fanfiction.net/s/1723301/1/

I'm also somewhat fond of my first attempt at the Five Things Which Never Happened... format, which I wrote about Warren:

http://www.fanfiction.net/s/1467137/1/