S/X recs

janedavitt asked for S/X recs. Several people posted links to their favorite S/X stories. These threads are always fun because they remind me of the great stories out there, some of which I read years ago. If you are like me and feel like revisiting some of these stories, or if you're wondering if your links still work, that thread is the place to go.
Of course you can also recommend great S/X fics that you read recently. You can find the thread here.