IMHO, the single-most underused HTML attribute. I get annoyed when I have to aim for the tiny box, instead of the nice long string of text next to it. Although, I can't count the number of times I was guilty of omitting it many moons ago.
<label style="padding:5pt"><input type=checkbox> this is label</label>
Here is how I started doing it:
<label for=abc><input type=checkbox id=abc ...> display alphabet<label>