We've added a beta branch to Steam. If you're using steam and you're sufficiently knowledgeable to switch to beta and you want to run the latest version of the game to test then you can switch to the beta branch.
So what is it and what does this mean? Well we'll use it to test things we want to test. Sometimes it will still login in the main normal server (like the current beta) and sometimes it will login to a beta server we've prepared.
Keep in mind that using beta software always carries some risk, so if you're not comfortable running betas and testing things then I'd advise you not do it. This is for people who are more tech savvy and who are comfortable with the kinds of issues that may come up.
If you are trying the beta and report a bug to us be sure to let us know the bug is in the beta branch and not the current stable release.