You are very lucky, it seems that you can pick the one you like. In order to help you with this task here are some links, that should help you to make an informed decision.
This distribution is based on the RedHat 7.3 SPARC distribution. The homepage is: http://www.auroralinux.org/ It is on the rise. There exists 3 mailing lists. You can join them at http://lists.auroralinux.org/mailman/listinfo
The latest version codenamed wombat can be downloaded on the ftp://auroralinux.org/pub/aurora/build-1.91/ ftp site.
Debian GNU/Linux runs on SPARC platform; you can find a lot of informations about this port at the http://www.debian.org/ports/sparc/ webpage
You can join the mailing list by sending a mail to < firstname.lastname@example.org > with subscribe as the subject.
This is a SPARC and SPARC64 distribution, more informations can be found in the Gentoo handbook at http://www.gentoo.org/doc/en/handbook/draft/handbook-sparc.xml
This distribution is based on the RedHat and is developed in Poland. They do have a SPARC 32 port. More information can be found at their homepage http://pld.org They also provide some mailing lists. More information can be found there: http://mail.pld.org.pl/mailman/listinfo/
The latest RedHat Linux distribution for the SPARC architecture is the 6.2; you can download it at http://www.linuxiso.org/download.php/243/zoot-sparc.iso for example.
There exists a mailing list about Linux and RedHat on SPARC Processors, you can access it there: http://www.redhat.com/mailman/listinfo/sparc-list
There exists a port for the SPARC architecture, more information is available at http://www.slackware.com/ports/sparc/
Actually, the SPARC-Linux Xterminal Package is not a distribution, it is a way to use a diskless SPARCStation as an X terminal. This is a a very convenient package. More information can be found at http://www.pucebaboon.com/SLXT/
The Linux Terminal Server Project (LTSP) is a convenient way to turn a SPARC or an Ultra SPARC workstation into a diskless workstation. The project's homepage is http://www.ltsp.org/
Step by step instruction to setup a client on an Ultra SPARC can be found at the following website: http://math.univ-lille1.fr/ltsp-sparc/
It is meant to be Slackware Compatible for SPARC. Its homepage is http://sourceforge.net/projects/splack
The SuSE Linux 7.3 distribution is quite complete, you have 5 ISOs to download. You can grab it at the following URL: ftp://ftp.suse.com/pub/suse/sparc
In the subdirectories you will also find bonus software such as KDE libraries and programms.
There is a mirror site located in Germany: ftp://ftp.gwdg.de/pub/linux/suse/ And, there are some extra files on the SuSE's server. You can join the mailing list by sending a blank mail to < email@example.com > More information can be found at: http://www.suse.com
This is a Japanese distribution and there exist a SPARC port. It is an rpm based distribution. From the documentation, it supports both SPARC 32 and SPARC 64. The documentation (in Japanese ;-) ) can be read online at http://www.vinelinux.org/sparc/2.1.5/install-vine-sparc-2.1.5.html