Apple released Developer Preview 5 of OS X 10.10 Yosemite on Monday. The new release includes a number of interface tweaks, new features in apps such as Safari, where Apple is testing some rendering abilities, and other minor changes.
Apple is also testing changes in Gatekeeper, Handoff, iCloud, iPhoto, Aperture, and Markup.
Developer Preview seeds are separate from Apple's public beta releases of Yosemite. Typically, multiple Developer Seed releases will occur between each public beta release. Developer Previews are only available through Apple's developer program.
Apple is expected to release the final version of Yosemite this fall.