An AMD64 compiler and kernel port are underway at the labs.
A CPU server kernel runs on Marvell's Sheevaplug and Openrd-client (both use the Kirkwood SoC) and another runs on the IGEPv2 (using the TI OMAP3530 SoC).
A version for the some iPaq models is also distributed; see Installation on Ipaq.
405EP (w/405GP port in progress) based on Inferno CerfCube 405 port
440GX (in progress)
Lucent use some PPC cards internally, the code for these has been left in the distribution to help others doing their own ports.
Some students at CMU worked on a port to the iMac with Professor David Eckhardt. Work on the port has (more or less) stopped. In its current state, the port uses OpenFirmware-based device drivers; significantly, it lacks one for ethernet. For more information, see http://www.cs.cmu.edu/~412/projects/9mac/ or contact Prof. Eckhardt.
Chris Collins is updating the 2nd edition sparc32 port to the 4th edition.
Tim Newsham has ported the both the plan9 compiler and kernel to the sparc64. The code for this port is available in /n/sources/contrib/newsham:
Supported Platforms - Sun Ultra2 Supported Hardware - HME 10/100 ethernet - HME/FAS366 SCSI - no booting from disk yet - no support for sunlabel disk labels - Z8530 uart serial portThe code was last updated in April of 2005; some work is needed to bring it back in sync with the current kernel tree.
The plan9 distribution runs on the DEC Alpha PC164, it is also believed to run on the LX164 and with some small modifications on the 500au. More recently a port was done to the Dec Personal Workstation, sd550 http://groups.google.com/group/comp.os.plan9/msg/e690ed5040ae91af though with no graphics support.
Only network boot is supported at present.
MIPS - The second edition supported various MIPS-based platforms, including several then-popular SGI machines. SGI has declined to allow the kernel source to be distributed (unlike the 2e Sun sources, which are available). Tim Wiess has discussed doing a modern port.
BeBox - Dr C H Forsyth did a port of the Second Edition to the BeBox; detailed info and an image of the system can be found in: http://verso.terzarima.net/plan9/soft/bebox.html
Next Cube - The second edition supported 68k based Next cubes used as teminals, this code has not been updated to the 4th edition.
Vax 750 - The earliest file server port. The compiler binary was recently found but the source appears to have been lost in the mists of time.
Hobbit - An CRISP cpu developed by AT&T, see the entry in the Wikipedia
Compilers also existed for several other architectures which are no longer distributed. Some, like those for the Intel i960 and AT&T 3210 CPUs, where only ever used to program peripherals and thus where not extensively tested. Others, like the AMD 29000, were used for other projects, like ports of Inferno. See 2c(1) for the list of currently distributed compilers.