[Excerpted from a 2002-10-05 mailing list post, updated.]

Taxonomy of distributions. Your options are:

o Aurora SPARC Linux. A separately maintained SPARC port of RH 7.3.
Very current.
http://auroralinux.org/

o Debian. Modern, highly maintainable, and my personal favourite
distribution -- but it's a bit different from what many are used to.
http://auric.debian.org/~bcollins/disks-sparc/current/doc/index.en.html
http://www.debian.org/releases/stable/sparc/install
http://www.debian.org/ports/sparc/ http://www.debian.org/ports/sparc64/

o Splack/Slackware. Slackware discontinued its SPARC port at
v. 7.1 (current is 9.0), but Splack is a separately maintained fork,
currently tracking Slackware 8.0, and is up to date.
http://www.netunix.com/splack.html

o PLD GNU/Linux. Produced in Poland. No ISOs; you have to do a
network installation. RPM-based, and probably another Red Hat fork.
Very current.
http://www.pld.org.pl/

o Fire Linux. UltraSPARC distribution derived from Linux from Scratch.
Very current.
http://fdragon.org/linux.php

o Gentoo, the most popular build-from-source distribution. Current.
http://www.gentoo.org/doc/en/gentoo-sparc-install.xml

o Rock Linux, a build-from-source distribution. Current. 64-bit
support is shaky.
http://www.rocklinux.org/projects/sparc/sparc.html

o Linux-Mandrake. SPARC port hasn't been updated since 7.1 (current
is 9.1), and would be a little long in the tooth but not too bad.
http://distro.ibiblio.org/pub/Linux/distributions/mandrake/Mandrake-iso/sparc/
http://sunsite.utk.edu/ftp/usr-436-1/Mandrake/Mandrake-iso/sparc/

o SuSE. A set of five ISO images is available for v. 7.3 (current is
8.2). Probably discontinued.
http://lists.suse.com/archive/suse-sparc/
http://sunsite.utk.edu/ftp/usr-436-1/suse/suse/sparc/7.3.cont/iso/

o TurboLinux. SPARC port was probably discontinued with the Workstation
v. 6.1 developers' release (current is 8.0?), but it's difficult to
tell with TurboLinux, many of whose operations keep disappearing into
Japan following renewed corporate follies.

o SCO Linux (formerly Caldera OpenLinux). SPARC port was discontinued
at v. 2.2 (current is 3.1.1), and will be ancient if you can find it.

o Red Hat Linux. The SPARC port was discontinued at v. 6.2 aka "zoot"
(current is v. 9), and will be ancient if you can find it.
http://www.geocrawler.com/lists/3/Red-Hat-Linux/85/0/
https://listman.redhat.com/mailman/listinfo/sparc-list
http://www.redhat.com/docs/manuals/linux/RHL-6.2-Manual/multi-arch/

o Vine Linux. A fork of Red Hat Linux 6.2 for SPARC, in the Japanese
language only.
http://www.vinelinux.org/

o Kondara MNU/Linux. A fork of Red Hat Linux 6.2 for SPARC, in the
Japanese language only.
http://www.kondara.org/

o UltraPenguin. The grandfather of all SPARC Linux ports, originally
based on a very old (5.x) Red Hat tree. Discontinued in 1999.
You can still find ISOs, but why?

Or you could install NetBSD (I have 1.5.2) or OpenBSD. The FreeBSD
SPARC port (sparc64-only) is said to be now beta-level reliable.
http://www.netbsd.org/Ports/sparc/
http://www.netbsd.org/Ports/sparc64/
http://www.freebsd.org/platforms/sparc.html

My summary:
One could make a good case for any of: Splack, Debian, Aurora, Gentoo,
NetBSD. Your call.