[marinabercea]

  * Timetabling or scheduling apps like Doodle.com - subjects pertaining to Graph Theory
  * Travel comparison sites that can find you the fastest flight route from A to B (and/or cheapest or some other type of constraints) - Graph Theory
  * LinkedIn network analysis apps such as Socilab.com - Graph Theory

Basically Graph Theory is extremely important, with applications in domains such as: networking, linguistics, (bio)chemistry, pathfinding & GIS, social network analysis, matching (dating apps, organ donation, medical residency matching) etc.

Edit: Another application is PCB design. The circuit to be printed is the graph itself, it should not allow edge crossings that would result in a short circuit.

[TheLogothete]

>less popular disciplines of CS

Graph Theory?

[marinabercea]

Considering the post submitter hasn't been more precise, I interpreted 'popular' as meaning less advertised or trendy. You can even use Graph Theory for image processing & analysis, yet another application.

[greydius]

Or more generally, topology. A graph is a 1 dimensional simplicial complex.