Hot on the heels of Devrim's Snapshot RPMs, I'm pleased to say we now have 'early experience' one-click installers available to download and play with the latest features committed to the PostgreSQL and pgAdmin source trees. We're aiming to update the installers periodically after interesting features are added - the first of which is Hitoshi Harada's SQL:2008 Window Functions patch for PostgreSQL.
You can download the installers for Linux 32 and 64 bit, Mac OS X 10.4+ and Windows.
Note that as development builds, these packages have had very little testing and should not be used on production systems.