The master distribution is 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 includes a MetaPost implementation as part of standard distribution currently featuring MetaPost 0.632 as of January 1997. This is source distribution which can be compiled not only on Unix platforms, but also for Amiga and Windows32.
An easy-to-install pre-packaged binary distribution of MetaPost 0.631 (as of August 1996) for a large number of Unix platforms is also available as part of Thomas Esser's teTeX distribution which is curently based on a patched version of web2c-6.1 but will eventually be updated to web2c-7.0 as well.
If you want a MacIntosh version, Thomas Kiffe's CMacTeX now includes MetaPost. OzTeX users now also have access to MetaPost. To get the pre-release version, contact Andrew Trevorrow.
Christian Schenk has ported MetaPost 0.632 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.