Tatlin is described as 'Multi-platform Gcode and STL viewer for 3D printing' and is an app. There are more than 10 alternatives to Tatlin for a variety of platforms, including Windows, Mac, Linux, Android and iPhone. The best alternative is F3D, which is both free and Open Source. Other great apps like Tatlin are Magics, Free View Expert, Netfabb and Viewshape.
Netfabb is not just a viewer, it provides mesh edit, repair and analysis capabilities to everyone already being or aspiring to become part of this fantastic, growing, creative, high-tech industry called Additive Manufacturing, Rapid Prototyping or 3D Printing.
The STL Viewer is FREE software for visualisation of STL or OBJ meshes created in packages such as 3D Studio Max, MAYA and Z Brush, modeled in CAD systems or generated from 3D digitalization of physical objects.