Snapshots are automatically built from the SVN trunk (development) repository sources. They are untested and might not work properly.
If long term stability and package availability is important, Attitude Adjustment should be used instead.
Attitude Adjustment (12.09 final) was released 25th April 2013
The snapshots can be downloaded from http://downloads.openwrt.org/snapshots/trunk. At certain times all platforms might not be available or up to date, due to builds failing.
Since the snapshots are updated frequently and development rapid at times, packages from the currently available build might not be compatible with previous builds. This is especially true for kernel modules, which require the exact same build to install. If incompatibilities occur, the only option is to flash the current firmware and try again.
The snapshots are built with a fairly minimal configuration, and by default does not contain a web interface, that has to be installed separately (luci.essentials).