Is it needed to "beautify" this Stack implementation?
class Stack


Stack (or any other collection) is a prime example of type that should be generic.

private int _length;


I think length is not a good name for this. You should rename it to something like _maxLength, or even better, _maxCount (see below).

Also, I'm not completely sure if this feature is necessary, but it could make sense (for example System.Collections.Generic.Stack&lt;T&gt; doesn't have anything like this, but System.Collections.Concurrent.BlockingCollection&lt;T&gt; does).

_length = 10;


I don't think 10 is a reasonable default here. It would certainly be unexpected to me. A much better default would be infinity. Or maybe don't have a default at all.

public List&lt;int&gt; Members


Unless absolutely necessary, you should never expose private implementation details like this. Using this member, anyone can do anything to the underlying collection (including adding items beyond the allowed maximum number).

If you want to be able to iterate over the items in the collection, you should implement IEnumerable&lt;T&gt;.

public int Length


The name Length makes some sense for arrays, but not so much for other collections. For consistency with other collections, the best name would be Count.

return (_members.Count == 0);


The parentheses are not necessary here.

if (_members.Count &lt; _length) _members.Add(member);
else throw new Exception("The Stack is full.");


You probably shouldn't write the body of if and else on the same line like this. Putting them on a line of their own will make your code clearer to read.

throw new Exception("The Stack is full.")


You shouldn't throw Exception directly, you should create your own type that inherits from Exception and throw that. Alternatively, you could use an existing exception type, like InvalidOperationException.

if (!IsEmpty)
{
    …
}
else throw new Exception("The Stack is empty.");


You could simplify this by reversing the condition and putting the exception first. This way, the main code will be less indented.

public partial class Form1 : Form


You should name your forms with more descriptive names, even if it's something as simple as MainForm.

lbStack


This form of Hungarian notation is usually frowned on. If you want to indicate that the variable is a list box, you can name is something like stackListBox.

(int)rnd.Next()


You don't need a cast here, Next() already returns an int.