I am curious if the setup for (co)homology theory appears outside the realm of pure mathematics.  The idea of a family of groups linked by a series of arrows such that the composition of consecutive arrows is zero seems like a fairly general notion, but I have not come across it in fields like biology, economics, etc.  Are there examples of non-trivial (co)homology appearing outside of pure mathematics?  

I think Hatcher has a couple illustrations of homology in his textbook involving electric circuits.  This is the type of thing I'm looking for, but it still feels like topology since it is about closed loops.  Since the relation $d^2=0$ seems so simple to state, I would imagine this setup to be ubiquitous.  Is it?  And if not, why is it so special to topology and related fields?

Robert Ghrist is all about applied topology:  Sensor Network, Signal Processing, and Fluid Dynamics.  (homepage:  http://www.math.upenn.edu/~ghrist/index.html ).  For instance, we want to use the least number of sensors to cover a certain area, such that when we remove one sensor, a part of that area is undetectable.  We can form a complex of these sensors and hence its nerve, and use homology to determine whether there are any gaps in the sensor-collection.  I've met with him in person and he expressed confidence that this is going to be a big thing of the future.

There are also applications of cohomology to Crystallography (see Howard Hiller) and Quasicrystals in physics (see Benji Fisher and David Rabson).  In particular, it uses cohomology in connection with Fourier space to reformulate the language of quasicrystals/physics in terms of cohomology... Extinctions in x-ray diffraction patterns and degeneracy of electronic levels are interpreted as physical manifestations of nonzero homology classes.

Another application is on fermion lattices (http://arxiv.org/abs/0804.0174v2), using homology combinatorially.  We want to see how fermions can align themselves in a lattice, noting that by the Pauli Exclusion principle we cannot put a bunch of fermions next to each other.  Homology is defined on the patterns of fermion-distributions.
