Fimfiction's story search is a powerful tool that has many functions in order to help you find stories you want to read.
Examples
#fluttershy #rainbow-dash #romance wilson:>80
Returns highly rated flutterdash fics.
#human #romance ( #fluttershy OR #rainbow-dash ) AND views:<1000
Returns stories with humans shipped with Fluttershy or Rainbow Dash that haven't been viewed much.
title:"my little dashie" -#twilight-sparkle status:complete
Returns completed stories with the phrase "my little dashie" in the title not tagged with Twilight Sparkle.
My Little Dashie OR Fallout Equestria
Returns My Little Dashie and Fallout Equestria stories.
While the tool is powerful, there are limits to what you can do with it in order to keep the site nice and fast. There is a maximum query "cost" and each element you add to the query contributes to it. The site will let you know if you go over this cost, but it shouldn't impede any normal usage.