When Cherie and Brian Lowe of suburban Indianapolis threw a party last spring, 150 well-wishers came bearing casseroles. This was no birthday or anniversary bash. Mr. Lowe, a lawyer, and Ms. Lowe, a stay-at-home mom, both 36 years old, were celebrating an accomplishment: paying off $127,482.30 in debt, mainly student loans. Partygoers came from as far as 200 miles away. Some had never met the Lowes but followed their quest through a blog called Queen of Free. Guests won prizes such as personal-finance books, museum passes and homemade cleaning supplies. One friend sang "Sallie Mae Is Not My Lender" to...