All source code and binaries are distributed under open-source licenses. The libraries are distributed under the LGPL, and the plugins and command-line utilites (flac and metaflac) are distributed under the GPL. If you would like to redistribute parts or all of FLAC under different terms, contact me.
Releases are first made through SourceForge and can be found here. For each version there is a source release and binary releases for Linux, Windows, Solaris, and Darwin (includes OS X). Winamp 3 note: the FLAC plugin for Winamp 3 is currently broken but you may have luck getting the version on RareWares to work.
RPMs can be found on Planet CCRMA and rpmfind.net.
Debian packages can be found here.
Mac OS X users, be sure to check out MacFLAC, a FLAC distribution specifically for OS X, including a .pkg installer and GUI front-end. Or, if you use Fink, you can try the FLAC Fink package.
IRIX users can find FLAC packages here.
Amiga users can find a pre-compiled FLAC package here.
You can also get the nightly CVS tarball here.