At present there doesn't seem to be a definitive list of the Linux ports that are in existence. The URL's below are simply the ones that people have submitted for inclusion in the FAQ. If you do know of a definitive list, please let the FAQ maintainer know.
On Intel platforms, VESA Local Bus and PCI bus are supported.
MCA (IBM's proprietary bus) and ESDI hard drives are mostly supported. There is further information on the MCA bus and what cards Linux supports on the Micro Channel Linux Web page, http://www.dgmicro.com/mca. Refer also to the answer for: "Where Is the Linux Stuff on the World Wide Web?"
There is a port of Linux to the 8086, known as the Embeddable Linux Kernel Subset (ELKS). This is a 16-bit subset of the Linux kernel which will mainly be used for embedded systems, at: http://www.linux.org.uk/Linux8086.html. Standard Linux does not run 8086 or 80286 processors, because it requires task-switching and memory management facilities found on 80386 and later processors.
Linux supports multiprocessing with Intel MP architecture. See the file Documentation/smp.tex in the Linux kernel source code distribution.
A project has been underway for a while to port Linux to suitable 68000-series based systems like Amigas and Ataris. The Linux/m68K FAQ is located at http://www.clark.net/pub/lawrencc/linux/faq/faq.html. The URL of the Linux/m68k home page is http://www.linux-m68k.org/faq/faq.html.
There is a m68k port for the Amiga by Jes Sorensen, which is located at ftp://sunsite.auc.dk/pub/os/linux/680x0/redhat/. The installation FAQ for the package, by Ron Flory, is at http://www.feist.com/~rjflory/linux/rh/.
There is also a linux-680x0 mailing list. ("What Mailing Lists Are There?")
There is (or was) a FTP site for the Linux-m68k project on ftp.phil.uni-sb.de/pub/atari/linux-68k, but this address may no longer be current.
Debian GNU/Linux is being ported to Alpha, Sparc, PowerPC, and ARM platforms. There are mailing lists for all of them. See http://www.debian.org/MailingLists/subscribe
One of the Linux-PPC project pages has moved recently. Its location is http://www.debian.org/MailingLists/subscribe. http://www.linuxppc.org, and the archive site is ftp://ftp.linuxppc.org/linuxppc.
There is a Linux-PPC support page at http://www.cs.nmt.edu/~linuxppc/. There you will find the kernel that is distributed with Linux.
There are two sites for the Linux iMac port: http://w3.one.net/~johnb/imaclinux, and http://www.imaclinux.net:8080/content/index.html.
A port to the 64-bit DEC Alpha/AXP is at http://www.azstarnet.com/~axplinux/. There is a mailing list at vger.redhat.com: ("What Mailing Lists Are There?")
Ralf Baechle is working on a port to the MIPS, initially for the R4600 on Deskstation Tyne machines. The Linux-MIPS FTP sites are ftp://ftp.fnet.fr/linux-mips and ftp://ftp.linux.sgi.com/pub/mips-linux. Interested people may mail their questions and offers of assistance to firstname.lastname@example.org.
There is (or was) also a MIPS channel on the Linux Activists mail server and a linux-mips mailing list. ("What Mailing Lists Are There?")
There are currently two ports of Linux to the ARM family of processors. One of these is for the ARM3, fitted to the Acorn A5000, and it includes I/O drivers for the 82710/11 as appropriate. The other is to the ARM610 of the Acorn RISC PC. The RISC PC port is currently in its early to middle stages, owing to the need to rewrite much of the memory handling. The A5000 port is in restricted beta testing. A release is likely soon.
For more, up-to-date information, read the newsgroup comp.sys.acorn.misc. There is a FAQ at http://www.arm.uk.linux.org.
The Linux SPARC project is a hotbed of activity. There is a FAQ and plenty of other information available from the UltraLinux page, http://www.ultralinux.org/.
The Home Page of the UltraSPARC port ("UltraPenguin") is located at http://sunsite.mff.cuni.cz/linux/ultrapenguin-1.0/, although the URL may not be current.
There is also a port to SGI/Indy machines ("Hardhat"). The URL is http://www.linux.sgi.com/.