The most popular is Daz3D.. the basic program is free but the add-ons can start climbing into the several hundred dollar range... You get models, clothes, textures, lights and backgrounds that put in a scene. Spend time posing and morphing the model, even more time trying to get textures of clothes and skin right, then be prepared to spend a lot of time fiddling with the lighting and trying to get a halfway decent render without turning your computer into a smoking husk.
Programs similar to Daz3D are Poser Pro, Maya, Carrara, Blender, MakeHuman, etc. Good examples of it are games by Rimyrr, Icstor, Mortze, PandP, etc.
TK17 (Sexvilla/Thrixx) doesn't have the morphing ability or the flexibility of Daz BUT it's much easier to get a model posed and ready, and you can also make videos quite easily so it's a faster process. As a bonus there's a huge amount of mods available at modsgarden for clothes, scenes, lighting configurations, sex positions (animated), and poses. A couple games that use it are "Beggar of Net - My New Life" and "The Twist".
Then there's Honey Select/Sexy Beach Premium Resort from Illusion. Similar to TK17 but with a different flavor and it's also fairly easy to use. It's also got really neat mods to improve the lighting of your scenes and giving your models some really neat textures. Look up Glassix, Cohabitation, and Lab Rats.
To recap.. you will have to pay something in both money and face a learning curve to make stuff in Daz, Poser, Blender, etc. But there is almost NO limit to what you can do if you invest some time in it.
You can find torrents of TK17, Honey Select, etc and get going on those much faster but the trade off is less flexibility.