I stopped hosting the master distribution at Version 0.632 and the last version I created was 0.641. This master distribution was available from CTAN under graphics/metapost.
The master distribution was intended for Unix and depends on the web2c package for Pascal-to-C translation. Warning: the installation process is more difficult than it probably shoud be.
As of version 7.0 released in February 1997, Karl Berry's web2c package included a MetaPost implementation as part of standard distribution. This is source distribution which can be compiled not only on Unix platforms, but also for Amiga and Windows32.
Various TeX and LaTeX system provides include pre-packaged binary distributions of MetaPost. One of them probably is teTeX and I assume there are a few others.
My original link for MacIntosh version was Thomas Kiffe's CMacTeX. OzTeX users also have access to MetaPost.
Christian Schenk has ported MetaPost to Win32. The port is part of MiKTeX.
Bart Childs has done a DOS version. Uwe Baumert's version for OS/2 is available from CTAN, and so is Piet van Oostrum's DOS version.