Perl 6 Compilers

Rakudo is copyrighted by the Perl Foundation, was born and raised in the Parrot virtual machine, and is actively developed here. Download or compile Rakudo.
Niecza is a compiler targetting the Common Language Runtime (.NET and Mono). Its goal is to explore the static aspects of Perl 6 and the possibilities for compile-time optimization.
STD is Larry Wall's reference implementation of the Perl 6 grammar, parser generator, and some of the semantic verifier. STD is an abbreviation of "Standard", as in "Perl6.0.0-STD". It is written in Perl 6, transliterated into Perl 5 by viv. It is also used by various phases of several other compilers (mildew, perlito, elf).
viv ("vi" to "v", 6 to 5) is a compiler that uses STD as its front-end. STD itself uses viv to build. It is actively developed along with STD.
Perlito, also known as MiniPerl6, is a subset of Perl 6, designed as a light bootstrapping language. You can try it online. It is actively developed here.
Pugs is written in Haskell. Maintenance releases for successive versions of ghc are published to Hackage. The most recent (6.2.13.20110519 as of this writing (2011-07-27)) was released 2011-05-19.