it really depends on how much you want to spend and what features you want. the most photoshop like program for 3D painting is Bodypaint3D from Maxon. This is what many of the big Production studios like Sony Pictures use. the price is pretty high though. Bodypaint has multiple material zone and full layer support. it paints in both projection and direct painting modes. another option is Deep Paint by Right hemisphere, this is probably the most popular 3D painting tools but its showing its age a bit and by some accounts Deep Hemisphere has stopped updating it, and its PC only its also fairly pricey it has pretty much all the options of Bodypaint (though you have to purchase the seperate Deep UV if you want to uv map in it. a third option is Modo 201, this is a full modeling Painting and rendering program. The painting and rendering features are brand new. again not inexpensive but for the price you get one of the best modelers on the market and probably the most innovative 3D paint system on the market. Zbrush has been covered by Moochie, its a great tool though I find its interface maddening personally. Another newcomer in the 3d Painting market is hexagon, very easy to use, but its painting features are fairly limited, only single layer painting, no projection painting, but it will do displacement painting which none of the previous tools except ZBrush does directly. Hexagon is also the cheepest of the bunch.