or your public library.
in my town they usually sell the magazines for 25 cents to a dollar. Sometimes they will let you fill a big for a dollar.

i've donated copies of backpacker to a scoutmaster friend of mine, and the rest usually go to the doctor's/dentist's office when i have my checkups.

Students majoring in art or design will happily take all sorts of old magazines off your hands. I made good use of my back issues of "Seventeen" in college (I pilfered the fashion spreads for my History of Costume studies), then gave some to my suitemate (a graphic design major) and donated the rest to the college's resource center.

Offer to donate them to an elementary school. School teachers often like to use magazines with lots of pictures for collage projects. (For example, children can cut out pictures of people and paste them inside a paper bus on the day they learn about bus safety, or they can use pictures of flowers for a lovely spring arrangement).

I used to donate mine to my local gym, lots of people read magazines on the ellipticals or stationary bikes

My Mom does that... She brings Enormous bags of mags she's collected from my other family members and her neighbors to all her and my Dad's doctors visits