I would try town meetings, schools, libraries, public institutions as such. You could ask your state department and ask if any need is out there for help.

There is an Arbor Day organization (national and local) with tree-planting activities. Try googling for details.

ask a local park district, a local nature center, a local audobon society chapter, a local TNC site, or a local chapter of the sierra club. they are usually always looking for volunteers. of course your specific request to "plant trees" may be only an occaisional need. sometimes they need "weed pullers" for garlic mustard control or sometimes they need "brush cutters" for honeysuckle or buckthorn control, sometimes they need someone to do some plant, butterfly or bird monitoring.

i am not real hip on the arbor day foundation because they have been known to send out alot of trees that are not right for the geographic locations.

Sticks,
We cannot tell what area you hail from ....rural, urban,or wild west, so it is hard to put a face on where you are coming from. As stated above, groups like The Nature Conservancy more often need weed pullers, seed harvesters, and brush removers.

definitely call the arbor day foundation. my experience was that they knew exactly what trees were right for my area and would not send them before the recommended planting date.

i just read a story about a family that is planting trees in all fifty states. they are planting one tree for each member of their family. i was pretty inspired. imagine planting a tree wherever you go. it sort of brings to mind Johnny Appleseed or his ilk.