Keyboard shortcut to sleep a Mac
If others do have a keyboard with an eject key, you can just press Command ⌘+Option ⌥+Eject ⏏.

Other shortcuts:

Control ^+Shift ⇧+Eject ⏏ - Put displays to sleep
Command ⌘+Control ^+Eject ⏏ - Restart
Command ⌘+Control ^+Option ⌥+Eject ⏏ - Shut down

If others like the OP have a MacBook Air or a MacBook Pro with a power key in place of an eject key, substituting the power key for eject should work in Mountain Lion and later versions of OS X, but not in Lion.

Since Mavericks, another option is to just press the power button or power key. It now puts a Mac to sleep instead of showing a dialog with different options.

If you don't have a keyboard with an eject or power key, you can use a private.xml like this with KeyRemap4MacBook:

&lt;?xml version="1.0"?&gt;
&lt;root&gt;
  &lt;item&gt;
    &lt;name&gt;test&lt;/name&gt;
    &lt;identifier&gt;test&lt;/identifier&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::S, VK_CONTROL, VK_OPTION, VK_COMMAND | ModifierFlag::NONE, KeyCode::VK_CONSUMERKEY_EJECT, ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L&lt;/autogen&gt;
 &lt;/item&gt;
&lt;/root&gt;


Or create a workflow like this in Alfred: