Since the Hamcrest matching library has been taken out of jmock and placed in its own package it looks to be generally useful for the application developer.  Hamcrest support has now gone into the new version of junit4.4, along with some other interesting features. 

The assertThat assertion now takes a matcher which provides a better alternative to assertTrue improving the error report when a test does fail.  Here are some examples:

import static org.junit.Assert.*;
import static org.hamcrest.CoreMatchers.*;
 
public class Junit4FeaturesTest {
 @org.junit.Test
 public void testArrays()
 {
    assertThat(new Integer[] {1,2,3,4,5},equalTo(new Integer[]{1,2,3,4,5}));
    assertThat(new Integer[] {1,2,3,4,5},not(new Integer[] {1,2}));
 }
}

If either of these tests were to fail the framework will now display the contents of the two arrays for inspection, avoiding the need for hand-crafted loops or helper functions. These are hamcrest matchers from the org.hamcrest.CoreMatchers, junit however provides some of its own matchers:

import static org.junit.Assert.*;
import static org.junit.matchers.JUnitMatchers.*;
 
import org.junit.Test;
 
public class Junit4FeaturesTest {
  @Test
  public void testSubstring()
  {
	assertThat("Hello World",containsString("Hello"));
  }
  @Test
  public void testCollections()
  {
	assertThat(java.util.Arrays.asList(1,2),hasItem(1));
	assertThat(java.util.Arrays.asList(1,2),hasItems(1,2));
  }
}

The use of this library may be quite a blow to other mocking frameworks. Using the hamcrest library means that JMock probably should be your mocking framework of choice: any custom Matchers you write will work with assert as well as jMock's expectations.