One of the major selling points for Windows 10 will be universal apps: code that can run equally well on a desktop, tablet or phone. Microsoft has just pushed out test versions of the tools you'll need to build those apps.
The Windows 10 developer tooling is available to anyone signed up for the Windows Insider program. It includes code for creating an adaptive user experience based on the device being used, and the ability to check if specific APIs are available.
Check the announcement post linked below for more details on the new tools.
Windows 10 developer tooling preview now available to Windows Insiders [Building Apps For Windows Blog]