What is a gravatar? - simply it is an image that appears next to your name when ever you comment on Gravatar enabled sites. It is very simple to set up and its free - have a look at the gravatar website.
You will need to install a plug-in for movabletype and wordpress sadly LiveJournal is not yet supported. For Blogger they suggest replacing the commenting system with HaloScan or blogkomm which do. Other modifications for other blog tools are given.
Wanna see it in action? Well it would seem there is only me and one other in the food/wine blog world that uses it. I see it all the time in other fields. HAve a look at http://www.spittoon.biz/archives/recommended_small_growers_champagnes.html
If you need some help with graphic design or how to modify your movable type template just ask.