Microsoft has released a new toolset to support developing applications on Windows 10. Titled Visual Studio Tools for Windows 10 Technical Preview, this toolset emphasizes developing Windows universal apps which can run unmodified across all devices (PC, phone, XBOX, etc.)-- the only requirement is that the device must be running Windows 10.
Microsoft's S. Somasegar notes in his announcement that not all devices can yet execute these apps created with this preview (in particular XBOX), but Windows 10 Phones can be simulated with the included emulator today. As part of the Windows 10 progression, all .NET apps delivered through the Windows Store will utilize .NET Native, which enables faster applications and less memory usage.
Developers looking to explore what Windows 10 features are available have several resources available to them. A general overview has been provided at the Windows Dev Center offers background on the various feature areas (media, networking, security, etc.) available for developers. Some of the interesting additions include the ability to get better details about the system battery (if applicable), drag-n-drop support between Windows RT apps and traditional desktop applications, and new APIs that provide Elliptical Curve Cryptography (ECC) support. There are 3 areas on a new Windows 10 section of GitHub which provide code samples for Windows 10 universal apps, drivers, and driver frameworks.
These tools require the latest release Visual Studio, VS2015CTP6, but this and the toolset can be found on the release page. Both a web-based and ISO copy of the files are available, but enrollment into the Windows Insiders program is required to download. Given that this is a preview, care should be taken to not install this in production environments.