The <label> element has been around for a while, but I still don’t see enough websites use it. In lets you make labels of checkboxes and radio buttons clickable.
Without <label>, you need to click directly on a checkbox in order to toggle it:
<input type="checkbox"> Some option
If you use <label>, the text after the checkbox becomes clickable:
<label><input type="checkbox"> Some option</label>
For styling via CSS, you may sometimes need another, more verbose, variant of this element: You can link a label to its control by referring to the control’s ID via the label’s attribute for (thanks to Evgeny Gorbachev for mentioning this use case for this attribute).