Has anyone done any volunteering in Madrid?

I work from home and go regularly to some local intercambios where I have met lots of cool people and have built some good relationships, but I would like to start meeting some more locals that are also helping to make a positive difference in the community.

Anyone know of a place where I can spend a few hours a week helping out and meet some cool people at the same time?

I was half way thinking of working somewhere for free just to meet more locals and practice my spanish.

Thanks!