Hi Roy,
First off if you save seeds from a hybrid variety, you will get seedlings different from the parent no matter what. The new generation could be good or could be bad, so unless you have room for experimenting, don't bother.
If you are growing one non-hybrid variety of each item, you can just save seeds and you can get the same as the parent. You could have trouble if someone is gardening close by like with corn it needs a certain amount of isolation. So you have to grow only one variety of corn.

The same type of item can cross with another of the same. Example Tomato 1 will cross with Tomato 2 to make offspring of different sorts, Pepper 1 will cross with Pepper 2 to make offspring of different sorts. But Tomatoes, though in the same family as Peppers, won't cross with Peppers.

Tomatoes and Peppers flowers don't need another flower for pollination. So if you bag the blossom before it can be possibly pollinated by an insect, it will pollinate itself. All the seed saved will be identical to the parent.

Squash and Pumkins need insect pollination or hand pollination. They can cross if their second half of their latin name is the same. Example Cucurbita pepo(many zucchini) and Cucurbita maxima(many pumpkin) won't cross with each other. Because I can't just bag a blossom, if I want to save seeds I just make sure I'm only growing one C. maxima variety.
Cucumber pollination works the same way as Squash.

I'm not sure if Sunflowers are self pollination or not.

Beans are self pollinating.

I hope I made a little sense, lol,
Remy

thanks for taking the time to explain it for me Remy,its very appreciated.

Roy

Most tomatos don't cross that easily unless you're growing more than one potato leafed variety. There might be some crossing but it's not all that often and still worth saving seed. If you're really needing pure seeds like to sell them it's one thing, but for home use it's usually fine-on tomatos. Peppers cross pretty easily. I disagree on saving seed from hybrids. I can save the first generation of offspring and usually not know the difference. Its when the save those seeds and generations after than the line goes downhill. Some hybrids are more stable than others.

re tomatoes:to add to remy's post you might want to visit the Tomatoe Forum's FAQ topic:"How Do I Prevent Tomatoes from Cross-pollinating....