Burning The Days (http://entangledsheets.livejournal.com/6039.html) by temporalranger is a lovely little slow build. There is sex fairly early, but the fic is basically 25k of relationship building. It's set across season 4/an alternate season 5 in which the boys prevented Lucifer from rising.

Summary:
When Dean and Cas show up in time to prevent Lilith's death, Team Free Will are all set to settle back into a life of saving people and hunting things - but nothing's perfect. Dean's a jerk who can't keep it in his pants; Cas feels there's plenty of room in his for the both of them. Sam's sure you can't be fuckbuddies with an angel and he's going to prove it - just as soon as he finishes scouring his eyeballs with soap.

But when Cas becomes Heaven's Most Wanted and the cops figure out the boys aren't as dead as they should be, Dean and Cas may prove to be both more and less obtuse than expected.