WordPress 3.0 Beta 1 is available for download at the following link:
http://wordpress.org/wordpress-3.0-beta1.zip

This is an early beta version, which means that it is the first version released outside the WordPress organization and prone to have a few undiscovered bugs. Beta testers willing to test the software for free should be aware that there are many new features available in 3.0. Check the lists below for reference.

Things you should be aware of:

• The custom menus system (Appearance > Menus) is not quite finished. In Beta 2, the layout will be different and a bunch of the functionality will be improved, but the WordPress team didn’t want to hold things up for this one screen. You can play with making custom menus, and report bugs if you find them, but this is not how the final screen will look/work, so don’t get attached to it.
• WordPress and WordPress MU (MultiUser) have merged. Still, that doesn't mean that you can all of a sudden start adding a bunch of new blogs from within your regular WordPress Dashboard. If you’re interested in testing the Super Admin stuff associated with multiple sites, you’ll need some simple directions to get started.
• There are a few small things in the UI that need to be beautified, such as implementing a new icon for the Super Admin section.

Things that need to be tested:

• The new default theme, Twenty Ten, including the custom background and header options.
• Custom Post Type functionality has been beefed up. It’s really easy to add new types, so go ahead and test it.
• WordPress MU users should test the multiple sites functionality to make sure nothing broke during the merge.

Testers are advised to use the wp-testers mailing list to discuss bugs you encounter.

If you already have a test install that you want to switch over to the beta, you can try the WordPress Beta Tester plugin.

Once you enable the plug-in it will by default switch your blog to the “Point release” stream and if you want to switch to the “Bleeding edge” stream then you will need to go to Tools -> Beta Testing page and configure the plugin there:

Please note: Once you have switched your blog to one of these beta versions of software it will not always be possible to downgrade as the database structure maybe updated during the development of a major release.