On Joel on blog, I found an interesting article posted: http://www.joelonsoftware.com/items/2010/03/17.html
It is a kind of introductory of Mercury source version control system.
First I thought we have already enough number of source version control systems such as Subversion, CVS, and AlienBrain.
The very first time I have heard about Mercury is from Google code. They actively supported it and they even had a video clip for it. I watched it but I didn't get how different it is from others.
Joel introduced it in a easy way, and I also think it is a very important big progress in software engineering. He also kindly made a tutorial of it: http://hginit.comI will try it on google code at a next personal project.
In a nut shell, Mercury is different from others in the sense that it stores "changes between versions" while previous systems store each version.
*PS: I don't see eclipse plugin for Mercury yet.