Ink til I'm Human (http://archiveofourown.org/works/919121) by templecat
No one expected this would be how he copes, least of all Castiel himself. But Dean seems to like it.

The Devil you know (http://archiveofourown.org/works/887657/chapters/1711516) Snarky Monkey
Castiel works at Purgatory Drop, a local tattoo parlor run by his good friend Balthazar. Unfortunately, for the last two years he's also worked alongside the attractive Dean Winchester.

Of course I second? Third? An Excercise in Worthless.

Of course there's the Tattoo Tag (http://archiveofourown.org/works?utf8=%E2%9C%93&work_search%5Bsort_column%5D=revised_at&work_search%5Bother_tag_names%5D=&work_search%5Bquery%5D=tattoo&work_search%5Blanguage_id%5D=1&work_search%5Bcomplete%5D=0&work_search%5Bcomplete%5D=1&commit=Sort+and+Filter&tag_id=Castiel%2FDean+Winchester) filtered through Cas/Dean in AO3 but those three are my favorites.