Официальный анонс:

Ruby 1.9.1 is released. This is the first stable release of the Ruby 1.9 series.

Ruby 1.9 is a new series of Ruby. It is modern, faster, with clearer syntax, multilingualized, a much improved version of Ruby.

Ruby 1.8 series has been used since 2003 and many great products were born on it.

Today, the Ruby 1.9 series starts its history as the 1.8 series did.

Please note that Ruby 1.8 still remains. 1.8.8 will be released this year.

You can read about major changes since 1.8.7 here

Так же в официальном анонсе перечислены URL для скачивания.