You can find it at: ftp://ftp.gnu.org/gnu/guile/guile-1.8.0.tar.gz. Its SHA1 hash is 22462680feeda1e5400195c01dee666162503d66.

The NEWS file is quite long. Here are the most interesting entries:

Changes since 1.6:
• Guile is now licensed with the GNU Lesser General Public License.
• The manual is now licensed with the GNU Free Documentation License.
• We now use GNU MP for bignums.
• We now have exact rationals, such as 1/3.
• We now use native POSIX threads for real concurrent threads.
• There is a new way to initalize Guile that allows one to use Guile from threads that have not been created by Guile.
• Mutexes and condition variables are now always fair. A recursive mutex must be requested explicitely.
• The low-level thread API has been removed.
• There is now support for copy-on-write substrings and mutation-sharing substrings.
• A new family of functions for converting between C values and Scheme values has been added that is future-proof and thread-safe.
• The INUM macros like SCM_MAKINUM have been deprecated.
• The macros SCM_STRINGP, SCM_STRING_CHARS, SCM_STRING_LENGTH, SCM_SYMBOL_CHARS, and SCM_SYMBOL_LENGTH have been deprecated.
• There is a new way to deal with non-local exits and re-entries in C code, which is nicer than scm_internal_dynamic_wind.
• There are new malloc-like functions that work better than scm_must_malloc, etc.
• There is a new way to access all kinds of vectors and arrays from C that is efficient and thread-safe.
• The concept of dynamic roots has been factored into continuation barriers and dynamic states.

(But what is Guile?)