What are some interesting problems in the intersection of Algebraic Number Theory and Algebraic Topology? I'm a beginning graduate student and while my background is primarily in algebraic number theory, I've found myself a bit smitten with the subject of algebraic topology recently after only having read some of Hatcher (Chapters 1 and 2 and I'm currently reading Chapter 3- and this is currently the extent of my topology background). 
I'm trying to get an idea of what sorts of problems lay at the intersection of these two fields, for now just so I have a direction of what sort of background I might want to be learning in the next year or so.
I realize this question is rather broad, but does algebraic number theory have any nice applications to topological problems? 
 A: Check out Vic Snaith's work on Explicit Brauer Induction.
A: Try "Primes and Knots" - By Toshitake Kohno & Masanori Morishita.
A: There are applications of the theory of cyclotomic fields to free actions of finite groups on $S^n$. The existence of such actions is tied to the class groups of certain cyclotomic fields $\mathbb{Q}(\mu_N)$ and their maximal real subfields $\mathbb{Q}(\mu_N)^+$.
You can find a brief introduction to this concept on p265 in Lang's Units and Class Groups in Number Theory and Algebraic Geometry: http://projecteuclid.org/euclid.bams/1183548780
Here are some of the relevant references he cites:
J. MILGRAM, Odd index subgroups of units in cyclotomic fields and applica-
tions, Springer Lecture Notes no. 854 (1981).
J. MILGRAM, Patching techniques in surgery and the solution of the compact
space form problem.
D. KUBERT, The 2-primary component of the ideal class group in cyclotomic
fields.
C. T. C. WALL, Classification of hermitian forms VI, Ann. of Math. 103 (1976)
pp. 1-80.
A: Chromatic homotopy theory is one such point of interaction between the two subjects.
The story of chromatic homotopy theory is a long one, but a version of the history might go as follows.  (This is highly abbreviated, revisionist, insufficiently referenced, and overlooks many aspects and contributions of many people.)


*

*Ordinary (co)homology theory is developed and turns out to be a useful tool.

*Later, certain "generalized" cohomology theories are developed, such as K-theory and bordism.  These are, in some sense, built out of ordinary cohomology, but some of them seem quite capable of sifting out interesting information, telling us geometric facts, or reassembling some nasty torsion information into something more accessible.

*Flipping roles, generalized cohomology theories can be studied in their own right.  They come from a category called the stable homotopy category (which is much like a derived category of chain complexes), and each of them can be determined by a certain amount of data involving cohomology operations.  Much of this data can be recovered by looking at how the generalized cohomology theory behaves on certain spaces (projective spaces and classifying spaces being the canonical examples).

*After a lot of hard work (with some of the bigger names including Adams, Milnor, and Quillen, though I am leaving a lot of important names out) it is discovered, starting from almost pure calculation, that the stable homotopy category has a connection to the category of 1-dimensional formal groups, via the study of characteristic classes.

*Further study affirms this connection.  Each generalized cohomology theory determines some amount of formal group data.  Certain theories that were particularly interesting turn out to have particularly interesting formal group data.  Certain computational tools have interpretations in terms of formal groups.

*Then - ! - making use of this interpretation systematically, via things like BP-theory and the Adams-Novikov spectral sequence, leads to better qualitative understanding of the stable homotopy category, new guesses about what phenomena can occur (e.g. the Ravenel conjectures), new techniques which are computationally useful, and new theorems (e.g. the solution of most of the Ravenel conjectures).

*Later, these things also find connections with mathematical physics, via a track through mathematical physics, string manifolds, modular forms, elliptic curves, and formal group laws.  This leads to the development of elliptic cohomology theories and topological modular forms.

*However, we still have very little understanding of why this connection arose in the first place, and most of the ways of showing that it exists at all are still through pure computation.  Constructive tools are still missing.
Here is a link to Lurie's recent course notes on the subject; Mike Hopkins has an ICM address on this topic which is quite nice; there are many other references.
A: This is something I just heard about yesterday. In this article
Nodal quintics in P^4.
B. van Geemen and J. Werner 
In: Arithmetic of Complex Manifolds; W.-P. Barth, H. Lange Eds. Springer LNM 1399, (1989) 48-59.
(find a copy here: http://users.mat.unimi.it/users/geemen/publ.html).
the authors compute the Betti numbers of certain smooth complex Calabi-Yau manifolds using a combination of techniques from algebraic topology and number theory. If I understand they first reduce the equations to some finite field, then combining the use of etale cohomology, the Lefschetz fixed point theorem and other things they reduce the problem to counting a finite set of points. 
A: Take a look at Machlachlan and Reid's book "The Arithmetic of Hyperbolic 3-Manifolds".  
Since finite volume hyperbolic structures are unique whenever an $n$-manifold ($n\geq 3$) has them, any invariants of the hyperbolic structure are invariants of the manifold.  Hyperbolic manifolds are $K(\pi,1)$-spaces, so they're not just diffeo/homeomorphism invariants, but invariants of the homotopy-type. 
A: Vandiver's conjecture (about class numbers) can be tackled through algebraic $K$-theory,
which is defined via algebraic topology: the conjecture is equivalent to $K_n(\mathbb{Z}) =0$ 
when $n$ is a multiple of $4$.  But that is a really hard problem.
A: The field of L-theory, the algebraic K-theory of quadratic forms, lies at the intersection of algebraic topology and of number theory. My impression is that it is an underpopulated discipline partially because it requires background in fields which most graduate students would think of as being disjoint. I think it is both deep and interesting.
A typical problem would be the calculation of a high-dimensional cobordism group (topological problem). You would show this to be isomorphic to a polynomial extension over the integers, and the actual computation would be to calculate the corresponding L-groups for the corresponding polynomial extensions over the rationals (number theory), and then localize to pass to results over the integers.
As a reference, I would recommend any book by Andrew Ranicki (High Dimensional Knot Theory is very nice, for example). See also this book review.
A: I think the whole field of anabelian geometry fits the bill, even if it's perhaps more focused on going the other way around (i.e. applying homotopy theory to number theory). Anabelian geometry is a 'program' launched by Grothendieck in his famous Esquisse d'un Programme, and is all about translating arithmetic geometric problems to problems in homotopy theory.
As an example of a specific instance of the anabelian philosophy, we have Grothendieck's celebrated 'section conjecture', which states (in one form) that for a 'nice' curve $X$ over a number field $F$, the rational points are in bijection with the sections of the exact sequence
\begin{equation}
1 \rightarrow \pi_1(X_{\bar{F}}) \rightarrow \pi_1(X) \rightarrow G_F \rightarrow 1
\end{equation}
where $G_F$ is the absolute Galois group of $F$ and $\pi_1$ is the algebraic (etale) fundamental group. In case the curve is over the complex numbers, the etale $\pi_1$ is the profinite completion of the regular fundamental group, so there is a very close connection to the classical stuff of Hatcher. The conjecture is still a wide open problem, but any proof would mean you could check something of number theoretic interest (existence of rational points on curves) by studying maps between certain generalized homotopy groups!
A: Étale cohomology?
