It’s a new 3D painting program made by Allegorithmic, a French company that may be familiar to you if you were ever researching available options for procedural texture generation software: MapZone, a free oldie-but-goldie, is a real gem, especially if you are on a tight budget (free, duh) and aren’t afraid of winging it in the absence of thorough documentation.
These days, Allegorithmic have expanded and improved on the innovative features of MapZone, and they have given a special name to their unique procedural texture format: Substances.
They have a whole line of software dedicated to working with Substances. Substance Painter is one of them, but you can also import any bitmap to use as a brush, alpha mask or stencil. It also has very neat dynamic particle effects! Think showering your model with rain or setting parts of it on fire, and getting realistic evidence of these processes onto your textures.
Generally, most people using Substances are game developers, but it’s not just for them. You can paint any models with it, export bitmaps and do whatever you want with them (I specifically asked on their forums, if it’s okay to make textures for models to sell or to give away).
You can try out Substance Painter here – you will need to create an account. I bought my licence when it was still in the beta, so I got a discount – and this promotion still lasts!