Another distribution for Ruby on Solaris 10 is Coolstack. Coolstack
provides an optimized Ruby version(and other stuff like Rails and
precompiled gems) so will most probably run your programs faster than
the other versions.
(Like Michael pointed out, Blastwave and Sunfreeware are options too -
Blastwave has a package retrieval client called pkg-get, which is
similar to apt-get. You have to download this first, and utter ‘pkg-get
install ruby’ - Sunfreeware uses plain old SVR4 packages).
Just FYI, I recently started a Ruby for Solaris project on Rubyforge
that focuses on the newer Solaris builds (ie., OpenSolaris and Solaris
Nevada)  - there are no Solaris 10 builds there though, given that
other places already deliver them. I’m probably going to put up Ruby
1.9-05 builds when I get time though.