Article Exotic $\mathbb{R}^4$ on Wikipedia says that there is at least one maximal smooth structure on $\mathbb{R}^4$, that is such an atlas on $\mathbb{R}^4$ that any other smooth $\mathbb{R}^4$ can be embedded into it. Is the construction of such a maximal exotic $\mathbb{R}^4$ explicit? Can anyone give a reference to the construction? What is a good source (or sources) with examples of exotic smooth structures on $\mathbb{R}^4$? Thanks.
The paper by Freedman and Taylor mentioned by Carsten Schultz (above or below) is indeed the place to find the explicit construction. Very roughly, the idea of the construction is as follows. Recall that a Casson handle is, among other things, a smooth 4manifold which is homeomorphic (but not diffeomorphic) to the standard open 2handle. It turns out that a countable collection of diffeomorphism classes of Casson handles suffice for solving a certain 5dimensional hcobordism problem. The universal $R^4$, call it $U$, is constructed by gluing together countably many copies of each of the Casson handles in the countable collection. Given an arbitrary smooth 4manifold homeomorphic to $R^4$, we can construct an embedding (but not a proper embedding) into $U$ using the fact that $U$ contains enough Casson handles to solve any link slice problem we might encounter along the way. Well, that was kind of vague, but hopefully not inaccurate. 


Another excellent reference is the book "4Manifolds and Kirby Calculus" by Gompf and Stipsicz. Section 9.4 is devoted to exotic R^{4}s. It describes many constructions of exotic R^{4}s and describes how to find the universal one. The book gives a beautiful overview of the complexity of smooth structures in dimension 4 and I highly recommend it. 


Google + Zentralblatt (Zbl 0586.57007) tell me: author="Freedman, Michael H. and Taylor, Laurence R.", title="{A universal smoothing of fourspace.}", journal="J. Differ. Geom. ", volume="24", pages="6978", year="1986", 

