Probably the most prolific author for Steve/Darcy on AO3 is Merideath, closely followed by nessimore. I'd also recommend "Five Things Darcy Loves About Working for SHIELD", by teand, which is a fic which follows Darcy joining SHIELD in the wake of the Chitauri thing (and has therefore been thoroughly and completely Jossed by all the subsequent films, but that doesn't stop it being a very good and funny fic in and of itself). If you can deal with a moderately angsty one, there's "Turn Off Trouble Like You Turn Off A Light" by blithers, where Steve and Darcy wind up getting blitzed and married in Vegas (yes, the necessary first step was Tony Stark creating something which could get Steve drunk!) - the story deals with the aftermath.
Let me know if you want any more recs - I've done my usual thing where I have bookmarks to just about every single bit of fanfic I've read in the Avengers fandom, and one of the sub-folders is just Steve/Darcy stuff.