sb_fag_ends = Don't know what I can say about this amazing place that I haven't said before, but if you don't read there, you are really missing out. For whatever reason, almost everything posted there is original, smart, gripping in some way (you'll laugh, you'll cry, you'll cringe), and well written. They are all short ficlets (although some can be strung together as stand-alone chapters that form a "verse"), so a great place to go for a quick read to fill some time. You may also discover some authors you didn't know about.
sb_ashtray - for fics that got too long for sb_fag_ends, or were finished too late for that week's prompts, or that otherwise weren't suited for that community. The same sort of quality, because it's mostly the same authors or others just as good.
fantas_magoria - A wonderful archive of fics written for every season of BtVS. They are on season 7 now, so I don't know where it will go from there, but the listing of fics already posted or linked to should keep anyone busy reading for years! Each round is a different episode and any fics that take place in the episode, or take off from that episode are eligible.
Lastly, seeking_spike - a community to help you locate fics, art work, etc you may have lost track of and want to reread.***Despite the Spike-centric name for this community, it's actually for any and all of the characters and/or ships in the Buffyverse. :) It is not a rec community, but one to help readers locate things they've already seen somewhere.