I am a huge fan of gmail. Ever since I got my very first invite, four years back from a really nice prof, I have completely stopped using the likes of yahoomail and hotmail.
To my surprise, I found the Labs option in my Gmail settings tab. It lets users add some experimental features to their gmail, try them and send feedback to google.
There are some really useful ones like Superstars, that lets you add additional stars to mark emails that need attention for specific purposes. There are some funny addons as well. For instance, Email Addict blocks your screen for 15 mins and prompts you to take a walk and get some real work done ;)
Overall, it's a nice feature added by google. And best part is, if any of the features break, they have given you an escape hatch. Simply click on this link.