Hi,
There is a tool called Blender. Here is a website with this tool:
http://www.blender.org/

My question is - can I use this tool in order to create a game which will be sold (not freeware, just commercial one)? This tool is free and open source software. However I heard that there is a law, some rules that if You use free software in order to create the commercial game You should share the code of game. If this is true, then the question is if I really have to share the code of game, because the only interaction between the game (the C++ code) and the blender is mesh hierarchy (X-files) loading into the game.

Thanks for any comments about this topic.
As far as maya 3D and 3D studio is concerned I know that those tools aren't free, that is why I would like to get familiar with blender and use it during my game development. However generally the funny story is that for example You can use maya3D or 3D studio and lie that You used blender. I don't know if someone check what tools You used when You had created game. Maybe there are some people on the forum who released in the past the commercial game (by commercial I mean that they sell it for 1 or a few $) and can share Your knowledge about that if someone (Inland Revenue or some other institution) verify the tools which You used in order to create the game???

You can use blender for commercial projects just fine. There is no law like the one you mention. Some software does have licenses that may require you to do that. This is on a software by software basis though, and blender has no such clause.

As the others have said, you'll be able to use Blender to create content for a commercial game no problem. Blender does have a game engine provided with it and you might have problems if you were to use that. But it uses OpenGL and, since this is the DirectX and XNA forum, I'm guessing you're not going to be using that. :)

Yes, that true, I am using directX, not game engine from blender. What about textures? For example I find texture of ground in one of the image in google. I see that this image come from some website (like private online shop or someone's private pictures gallery in facebook). I take this image, cut off the interesting me texture, fit the length into 256x256 and what do You think about it? It seems that acc. to law I cannot do that, but there is almost 0 percent chance that the person from which I use the texture find that I use his texture. But let's assume that the person use my games and find that one of the texture come from his private photo. What negative consequences I might have? We are talking here about 10-100$ of compensate or thousands of dollars?

Thousands of dollars, plus highly unethical. Do NOT do that.

If you want to use an image you find online, ask the copyright holder for permission. This is typically the person who maintains the page, or if not, that person can direct you to the copyright holder. Tell them how you'd like to use it, and what, if any, pricing you intend to charge for the final product, and if you can, what kind of sales you expect.

You might be surprised how willing some people will be to let you use their stuff. Some may want to charge a flat fee or some kind of royalty.

Another option is to but one or more texture packs. There are a number of websites that sell graphic assets for more or less reasonable rates.

And finally you can always download gimp and start going through online tutorials so you can make your own.

Following online tutorials for blender is also a good idea. That piece of software can do some pretty amazing stuff.

And if the person agrees, the agree in e-mail is enough (there isn't a need any of formal document)?

And what about .X file exporter add-on from blender? I know that there is default add-on in Blender, however someone else created better addon for blender which exports mesh from blender as .X file. I see that his license is GPL. As You know all SW which is based on GPL product must be GPL product as well. Does it mean that I cannot use this add-on script (so I have to use worse default one) if I want to sell my product?