Add-ons for your Gmail?

I just noticed Google have added a new feature to Gmail today. It is some sort of an add-on thing, but Google call it Gmail labs. Look at the top right of your Gmail window, click on 'Settings'. Then at the tabs, choose 'Labs'. I have provided a screenshot of it below.

So far there are just twelve experimental features. None I found which is useful or interest me though, except for 'Custom date formats' feature. Go try it out.