Three obvious situations where you need it:

• Calling another constructor in the same class as the first part of your constructor
• Differentiating between a local variable and an instance variable (whether in the constructor or any other method)
• Passing a reference to the current object to another method

Here's an example of all three:

public class Test
{
    int x;

    public Test(int x)
    {
        this.x = x;
    }

    public Test()
    {
        this(10);
    }

    public void foo()
    {
        Helper.doSomethingWith(this);
    }

    public void setX(int x)
    {
        this.x = x;
    }
}

I believe there are also some weird situations using inner classes where you need super.this.x but they should be avoided as hugely obscure, IMO :)

EDIT: I can't think of any examples why you'd want it for a straight this.foo() method call.

EDIT: saua contributed this on the matter of obscure inner class examples:

I think the obscure case is: OuterClass.this.foo() when accessing foo() of the outer class from the code in an Inner class that has a foo() method as well.