The semi-official site of packages is http://www.slackbuilds.org, but compiling them can be difficult.
By using Blackpkg http://sourceforge.net/projects/blackpkg/files/blackpkg-1.1-noarch-Sbo.tgz/download you can install packages of the repository in just one line of command, and set flags like the processor.
Example installing Google-Chromium. ( -march=native to compile natively for my CPU):
# export SLKCFLAGS="-O2 -march=native"
# blackpkg -y libevent chromium
Installing ExtremeTuxRacer with all 3D power of CPU!
# export SLKCFLAGS="-O2 -march=native"
# blackpkg -y extremetuxracer
