Some old favourites I keep going back to:

Five Times Clint and Natasha Slept Together (and the First Time They Slept Together) (http://archiveofourown.org/works/390944/chapters/641476) by eiluned
The Five Times Clint Barton Proposed to Natasha Romanov (And The One Time She Said Yes) (http://archiveofourown.org/works/330892) by eiluned
Five Dates... (http://workerbee73.livejournal.com/139874.html) by workerbee73
Keep Marrying You ('Til I Fall in Love) (http://archiveofourown.org/works/469175) by Della19
I am the heart that you call home (http://bob5fic.livejournal.com/65330.html#cutid1) by bob5fic
Lifeline (http://archiveofourown.org/works/413650) by sarea
First Name Basis (http://archiveofourown.org/works/408668?view_adult=true) by SugarFey
Assumptions (http://sandbattles.livejournal.com/9756.html#cutid1) by jacedesbff
Of Languages (http://www.fanfiction.net/s/8138868/1/Of_Languages) by QueenRiley
Find My Way To You (http://archiveofourown.org/works/376392?view_adult=true) by eiluned