fitting free QFTs into the Haag-Kastler algebraic formulation Has the free Klein-Gordon quantum field theory been fitted into the
Haag-Kastler algebraic framework?  (Actually, John Baez told me "yes", and he should know.)  If so, can you describe the basic
strategy and/or give pointers?
The same question for the free Dirac field theory.
 A: Yes, these are the standard examples. Some references are colleced here.
For a quick review/survey see for instance slides 11-17 in


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*Edison Montoya, Algebraic quantum field theory (2009) (pdf)


Discussion of the free scalar on Minkowski and curved spacetime is around section 3.2 of


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*Romeo Brunetti, Klaus Fredenhagen, Quantum field theory on curved spacetimes (arXiv:0901.2063)


Discussion of the Dirac field and its deformations is for instance in 


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*C. Dappiaggi, Gandalf Lechner, E. Morfa-Morales, Deformations of quantum field theories on spacetimes with Killing vector fields, Commun.Math.Phys.305:99-130, (2011),  (arXiv:1006.3548)


There are many more, just chase references. If any example at all is discussed, it is the free scalar field. That's what motivated and instructed much of the theory. The art is to go beyond that example.
A: As Urs mentioned in his answer, free fields really constitute the guiding examples for the various axiomatic systems of QFT. The construction of free fields in Minkowski space is a standard part of the theory, though it may take some digging to find the precise reference where it is checked that such a construction satisfies the desired set of axioms. In particular, for the Haag-Kastler axiom system, once the theory is built on all of Minkowski space, one must show injectivity and isomorphism of the algebras localized in subsets of Minkowski space, with respect to appropriate inclusions of these subsets.
Constructions of various free bosonic and fermionic fields, including the specific cases of scalar and Dirac fields can be found in these classic references, in somewhat varying degrees of detail:


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*Baez, J. C., Segal, I. E., Zhou, Z., Introduction to Algebraic and Constructive Quantum Field Theory (Princeton, 1992)

*Wald, R. M., Quantum field theory in curved spacetime and black hole thermodynamics, (Chicago, 1994).
There are two main stages to the construction. One has to build the linear space of solutions as a symplectic manifold (that's the classical phase space). Then one has to turn the algebra of functions on this space into a non-commutative $C^*$-algebra of quantum observables (that's quantization). Since the theories in question are linear, once the necessary functional analysis is in place, this is done by an infinite dimensional version of how it is done for a simple harmonic oscillator. For fermions it's fairly straightforward. For bosons one has to use the intermediate trick of working with the algebra of bounded functions generated by exponentiated smeared fields (that's the Weyl algebra). The actual unbounded operators representing smeared fields are constructed by taking derivatives of the elements of the Weyl algebra, once a representation has been chosen.
The same steps appear also in the work on QFT on curved spacetime, where different references described the individual steps in varying levels of detail. To get something like the Haag-Kastler axioms out of the latter constructions, one simply has to restrict onself to spacetimes consisting of causal-diamond shaped subspaces of Minkowski space. Classic references for particular field theories include:


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*Dimock, J., [Dirac field], Trans. Am. Math. Soc. 269, (1982) p.133 (1982)
http://dx.doi.org/10.1090/S0002-9947-1982-0637032-8

*Dimock, J., [Maxwell field], Rev. Math. Phys. 4 (1992) p.223
http://dx.doi.org/10.1142/S0129055X92000078

*Furlani, E.P., [Proca field], J. Math. Phys. 40 (1999) p.2611
http://dx.doi.org/10.1063/1.532718
The biggest technical sticking point in these references is how they treat (or don't treat) non-compact Cauchy surfaces.
The modern generalization of the Haag-Kastler axioms to arbitrary globally hyperbolic Lorentzian spacetimes are the Brunetti-Fredenhagen-Verch (or Locally Covariant Quantum Field Theory) axioms. Here's a couple of modern references that give the construction of free fields (subsuming most of the above particular examples), in great mathematical detail, that fits directly into this framework:


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*Bär, C., Ginoux, N. & Pfäffle, F., Wave Equations on Lorentzian Manifolds and Quantization, (EMS, 2007).
http://dx.doi.org/10.4171/037 http://arxiv.org/abs/0806.1036

*Baer, C. & Ginoux, N., Classical and quantum fields on lorentzian manifolds (2011)
http://arxiv.org/abs/1104.1158
(Note for the cognoscenti: in this restricted situation, quantization does happen to be a functor!)
Of course, much more literature can be found by digging backward and forward in the citation network starting with the above references.
