Mainly I've got multiple browser windows open with way too many tabs. I use a plugin to prevent them all from being actively loaded, but still have a ton active at any one time.

I also usually have one or more Excel windows open for data analysis, possibly Powerpoint or Keynote, and then some other lightweight stuff (Spotify, Sublime, etc.).

I also am most frequently using this across two 27" Thunderbolt displays (and if I'm desperate for a 3rd, I can open the MBA screen in addition).

I'm not a developer, so my needs may be less intensive than yours.