Math lesson plans can center around any of these service learning projects:
Nothing But Nets--This is a program that has been receiving a lot of attention lately because Ashton Kutcher and CNN have been racing to get 1,000,000 followers on Twitter to help raise money for Nothing But Nets. Ashton Kutcher won the race, but your students can help fight malaria too by raising change to buy mosquito nets for families in third world countries. The nets are easy to purchase through the Nothing But Nets website.
UNICEF has a program in October where children can collect change to help UNICEF programs while they also go trick-or-treating. UNICEF has several programs all over the world, and their website has resources for teachers and students.
Local community programs around your school will benefit from raising money. For example, you could create your own penny containers and raise money for your local animal shelter. You could extend this service learning project into an actual field trip to a pet store to purchase products and then take them to the animal shelter as a class.