Asymptotics for the number of abelian groups of order at most $x.$ The number of abelian groups of order $n$ (call it $a(n)$ is a studied subject (see http://oeis.org/A000688), but I can't seem to find any asymptotic results. Obviously, there is no asymptotic for $a(n)$ itself (it is much too irregular), but there should an asymptotic for $\sum_{n\leq x} a(n),$ but I can't seem to find a reference.
Roberto answered the above, but another question is whether one has any distributional results (how high are the maxima, is there a limiting distribution, etc).
 A: One reference where the asymptotic result you are asking for was first established (I think), as well as some reasonable growth estimates for $a_n$, is
D.G.Kendall and R.A.Rankin, "On the number of Abelian groups of a given order", Quart. J. Math., Oxford Ser. 18 (1947), 197–208.
(full text: http://qjmath.oxfordjournals.org/content/os-18/1/197.full.pdf)
A: According to http://mathworld.wolfram.com/AbelianGroup.html it is a theorem of Srinivasan (1973), click on the link for details.
A: The problem was studied quite a bit; a complete summary will be complicated to give (in any case I cannot). A standard reference for classical results on this is A. Ivić "The Riemann Zeta-function: Theory and Applications" (1985); it seems there is a recent Dover edition. Ivić has various papers on this problem, too. The book has a chapter on this subject (14.5 Non-isomorphic abelian groups of a given order). At the moment I cannot recall what exactly is in there.
Some information related to this type of problem:
The correct asymptotics for $\sum_{n\in \mathbb{N}} a(n)$ were obtained by 
Erdős and Szekeres (1934), they proved: 
$$\sum_{n\le x} a(n) \sim A x + O(\sqrt{x})$$
with $A= \prod_{n \ge 2}\zeta(n)$. 
By now there are more precise results known. For example  the paper by Kendall and Rankin (1947), mentioned in another answer, gives $Ax + B x^{1/2} + O(x^{1/3} \log x )$, and developments continued, like the result mentioned by Roberto Pignatelli.
The to my knowledge latest improvement there is by Sargos and Wu (2000)
$$Ax + Bx^{1/2} + Cx^{1/3} + O(x^{55/219} (\log x)^7 )$$
The first to get the third term of the main term was Richert (1952). 
For the historical development of the error see the introduction of a paper by  Calderón (2003).
Related questions were also studied. For example,  Kendall and Rankin (1947) showed that the "local densities" of $a(n)$ exist, that is   $\sum_{n \le x, \, a(n)=k}1 \sim d_k x$ (good error terms are known). 
There are also estimates "in short intervals" so on $\sum_{ x \le n \le y, \, a(n)=k}1$.
For two recent papers on this see: 
Emre Alkan, On the enumeration of finite abelian and solvable groups, J. Number Theory 101 (2003), no. 2, 404--423.
Ekkehard Krätzel, The distribution of values of the enumerating function of finite, non-isomorphic abelian groups in short intervals, Arch. Math. (Basel) 91 (2008), no. 6, 518--525.
The introductions and references there will lead to various additional paper. (The first is freely accessible.) 
References
C. Calderón:  Asymptotic estimates on finite abelian groups. Publications De L’institut Mathematique, Nouvelle série, 74, 57-70 (2003).
P. Erdős, G. Szekeres: Über die Anzahl der Abelschen Gruppen gegebener Ordnung und über ein verwandtes zahlentheoretisches Problem (in German), Acta Litt. Sci. Szeged 7 (1934), 95--102; Zentralblatt 10,294. Free PDF
D. G. Kendall and R. A. Rankin, On the number of Abelian groups of a given order, Quart. J. Math., Oxford Ser. 18 (1947), 197--208.
Hans-Egon Richert, Über die Anzahl Abelscher Gruppen gegebener Ordnung. I, Math. Z. 56 (1952), 21--32.
P. Sargos and J. Wu, Multiple exponential sums with monomials and their applications in number theory, Acta Math. Hungar. 87 (2000), no. 4, 333--354. 
