I posted this to HN and I realized it might be useful for other people. If at least one person who wasn't aware of fan-fiction discovers it as a result, this has served its purpose.
For Buffy fans, TTH has a huge number of stories, both in-universe and crossovers with other worlds. Speakertocustomers, Becuzitswrong, Cordyfan, DianeCastle and Hotpoint are a few of my favorite authors. Hotpoint's crossover between X-COM and SG-1 is absolutely amazing.
What I normally do is I download the stories in the .mobi format - TTH and AO3 have that feature in the site, and for the others you can use the FanFicFare plugin for Calibre - and then upload them to my Kindle. This allows me to read in bed, which is bad for my sleep but I like it :)