VTFLib provides a C and C++ API that, with a few simple functions, can open and save VTF and VMT files, providing access to all known features. The library functions independent of Steam, allowing third party applications to use the library without Steam present or running on the target system.
VTFLib includes two GPL example applications, VTFCmd and VTFEdit. VTFCmd is a C command line frontend for VTFLib that can create VTF and VMT files from various source formats. It is similar in functionality to Valve's Vtex Source SDK utility, but offers a lot more control. VTFEdit is a C++ .NET graphical frontend for VTFLib with viewing and creation capabilities. Both VTFCmd and VTFEdit support several source image formats, including, but not limited to BMP, DDS, GIF, JPG, PNG and TGA.
VTFLib uses the nVidia DXTn library at the heart of its DXT image processing and is able to produce compressed images of a much higher quality than the default SDK Vtex tool.
As a side project, VTFLib co-author, Neil Jedrzejewski has produced a version of VTFLib for Visual Studio 2005 which can be compiled as both 32-bit or 64-bit. Although not merged into the current version or released publically, anyone in need of the 64-bit version for their project should contact him.