If you are comfortable with PHP and would like to participate in the testing portion of our development cycle and report bugs you find, beta releases might be for you.
By their nature these releases are unstable and should not be used any place where your data is important. Please backup your database before upgrading to a beta release. To hear about the latest beta releases your best bet is to watch the development blog and the beta forum.
You can find the latest beta releases on the Beta Releases page.
Development of WordPress moves fairly quickly and day-to-day things break as often as they are fixed. This high churn is part of our development process that aims to produce the most stable releases possible.
If you would like to be part of this process, the best place to start is the Beta Testing Handbook.
You can download the latest nightly release here: wordpress-latest.zip.