<p>It seems like Groovy was forgotten in this thread so I'll just ask the same question for Groovy.</p>

<ul>
<li>Try to limit answers to Groovy core</li>
<li>One feature per answer</li>
<li>Give an example and short description of the feature, not just a link to documentation</li>
<li>Label the feature using bold title as the first line</li>
</ul>

<p>See also:</p>

<ol>
<li><a href="https://stackoverflow.com/questions/101268/hidden-features-of-python">Hidden features of Python</a></li>
<li><a href="https://stackoverflow.com/questions/63998/hidden-features-of-ruby">Hidden features of Ruby</a></li>
<li><a href="https://stackoverflow.com/questions/161872/hidden-features-of-perl">Hidden features of Perl</a></li>
<li><a href="https://stackoverflow.com/questions/15496/hidden-features-of-java">Hidden features of Java</a></li>
</ol>

Using hashes as pseudo-objects.
```
```
def x = [foo:1, bar:{-> println "Hello, world!"}]
x.foo
x.bar()
```
```
Combined with duck typing, you can go a long way with this approach.  Don't even need to whip out the "as" operator.