One of the first and most famous HDM crossovers is Red's Theory and Practice (http://archiveofourown.org/works/232514).

Asteroidea (http://archiveofourown.org/series/5911) by etothepii is well worth reading.

snake_easing was writing a very interesting series called Playing Human, but appears to have locked down their fanfiction. I can probably hook you up with a file of it.

dauntdraws did lovely fanart for the above three stories of John with his respective daemons. You can find it HERE (http://daunt.tumblr.com/post/1599212465/daily-sketch-blog-day-1-4-hours-okay-so-spent).

blind_author does only update infrequently, but I have every confidence they'll finish Republic of Heaven; they've put a lot of work into it.