Paul Graham's official distribution started it all, of course. This version is implemented in MzScheme and translates Arc into Scheme code. The writeup on Arc Internals provides more information on how it is implemented.
The Arc Forum is the site of most of the unofficial Arc development effort. The "Anarki" branches of Arc on github consist of a "stable" bug-fixed version of official Arc, and a more exploratory version with many modifications. (For information on using github, see Brief Guide to Anarki and Git.
Anarki includes "ac.sbcl.lisp", an implementation of Arc in SBCL. It implements closures using CPS (Continuation-passing style) transformations, but doesn't include threads or networking according to the brief description.
Rainbow (l'arc en ciel in French) is an implementation of Arc in Java. It is fairly complete, providing continuations and tail-call optimization according to the description. It is available at github.
An Arc compiler for .NET has been developed, based on the MBase compiler prototyping framework. The description states that it implements a subset of Arc, but is much faster than standard Arc. It is available from Meta Alternative.
Currently under active development is "arc2c", which compiles Arc to C. Current status is at arclanguage.org. This compiler is intended to be a full, high-performance implementation of Arc. It uses CPS transformations to implement continuations. It uses the Boehm garbage collector. arc2c is available at github.
CL-Arc is a proposal to compile Arc to Common Lisp; work hasn't started on it yet.
Many of these Arc implementations actively welcome participation. The future is likely to hold continuing improvement in the quality, functionality, and performance of Arc implementations, as well as surprising and unusual variants.