Do it For Free. Idea of site

Once again I have a weekend. Weekend when I want to do some stuff which will be helpful for someone.. stuff which will train my skills, my brain. I don't know what to do.. really.

So question: are there some sites where listed small jobs which I can do for free? If not, then it's a good idea for start-up. I think I am not only who have a free time, not need money and want to help to someone who don't have money but need some stuff.

If such site is already exists please provide me link, if not then this is my small view of this site:

- site has a list of small jobs with tags and categories

- site has a list of people who want to do something for free, every man has listed skills he have

- People can group to do some job

- Site has ratings of best free mans

I'm a web-developer and know that creating of such site will not take more than 3-4 hours, but it could be very useful..

What do you think?