Plan 9 - get an ISO and run Plan 9 on several hardware platforms (as usual, x86 is the initial path of least resistance), or in a virtual machine.
Inferno - itself a VM with software-protected processes that runs atop both common operating systems and on hardware, even hardware without an MMU. Inferno revolves around Limbo, a type-safe language in which the system and applications for it are implemented.
Acme-SAC - Caerwyn’s “acme stand-alone complex” is Inferno repackaged with acme(1) as user interface. If you’re like roughly 100% of people running a common operating system, this is probably the easiest way to see some nifty Plan 9/Inferno action. While acme provides the entire interface, a complete Inferno environment is available. Acme-SAC currently runs atop Linux, Mac OS X, and Windows.
Plan9port - Russ Cox’s 9 environment - not emulated - for Linux-Unix systems (I like someone else’s term, lunix). P9p includes a bunch of Plan 9 programs ported to run on lunix, as well as libraries and development tool front-ends to create software based on ported Plan 9 facilities.
Libixp - a 9P client/server library for POSIX-ish systems, for linking into your lunix programs. (9P is the unifying Plan 9 file protocol, called Styx in Inferno contexts. See here and here.)
Web9 (including PHP9P, PHP bindings to libixp, above) - making 9P available for web application development.
This is no exhaustive list; there are a number of 9P implementations.