The latest pre-release build can usually be found here.
Added for emphasis: Prerelease software should be expected to have bugs.
The purpose of a public alpha/beta/RC process is to help identify bugs in a real-world context. Prerelease builds will contain experiments, tests, and lots of new code. If you come across a bug, please report it along with a recipe to reproduce.
Please post any comments and especially bug reports about pre-release builds in this forum. It would probably cause less confusion if we keep discussion of features and bugs in the main forums, and on outside forums, limited to the current release build, and talk about the new stuff here.
Pre-release and especially builds marked "alpha" or "test" can have unexpected behavior! Best practice is to back up your entire settings folder (the folder containing "reaper.ini") and not save any non-backed-up critical projects while using a pre-release build.
Having said that, if you don't get along with a particular prerelease build, you can simply install the latest official Reaper release right over it and be back to normal in a matter of a few minutes.