I have been volunteering while traveling and have found the volunteer experience so drastically different from organization to organization.
Below are some organizations I would recommend and are free except for WWOOF which is a membership fee. 

Sonati - Leon & Esteli, Nicaragua (www.sonati.org) Free volunteer opportunities
Sonati works to provide free environmental activities funded by their hostel and tour agency.  They will give you a free bed in a shared dorm in exchange for either working in reception.  You can also volunteer as a trekking guide if you can stay 3 months (and food is provided on treks).  You can also volunteer for environmental education or with art helping to create murals and other projects using trash (they do not provide housing for these roles but do not charge any fees)  You can also propose a new project or they can help connect with you with schools or other interests you have.  
Oaxaca city Mexico (www.oaxacastreetchildrengrassroots.org/)   
I highly recommend this organization.  Its free to volunteer and there is no commitment.  Spanish is helpful but not required.  I recommend longer stays and of course bringing supplies such as school supplies, games, childrens books in spanish, and art supplies or other donations if you have room in your suitcase.

WWOOF www.wwoof.org/  - I recommend wwoofing though each place is different, I have had amazing experiences and disapointing ones and also find that many of the farms listed dont reply back.  I suggest reading reviews on the farms before going.  

www.couchsurfing.org/  Although this is not a volunteer opportunity per se, I recommend people to sign up and host people, its a great way to meet people from other cultures by having them camp out in your living room, extra room etc.  Its a way to learn about other cultures without having to leave your own home.

Condor Trekkers- Sucre, Bolivia (www.condortrekkers.org)
Located in Sucre Bolivia this a ecofriendly non profit trekking agency with amazing people.   Volunteer work can include helping in the office, preparing for treks, helping on treks and cleaning up after treks as well as many side projects.

Inti Magazine - Sucre Bolivia (http://inti-revista.org/about-us/)
Magazine helping working children by having them help create and sell a magazine where the profits go directly back to the children.  No minimum time to volunteer and no fee.

Banos after school program (http://artedelmundoecuador.com/?lang=es)
most volunteers live in the volunteer housing which helps to cover the cost of renting the building for the afterschool program.

Volunteer on a coffee farm in the country.  They are wonderful generous people and provide a bed and 3 meals.   Website: http://edimerome.blogspot.com/

Work exchange at a hostel - this is one of many hostels that offer a work exchange and will give you a free bed and maybe food if you do some work for them.  This is the only work exchange at a hostel that I have done but it was a good experience and I loved the town.  At the time they required a two week committment.   website http://www.lakeviewhostel.com/

Organic farm 

Let me know if you have opinions on other volunteer or travel opportunities