I have been taking Spanish classes at Spanish School Instituto Intercultural in Mendoza, Argentina. Although they offer lessons for several different languages, Spanish and English are definitely the most popular, which certainly makes sense being that we're in Argentina.

The prices are reasonable and the teachers are fantastic. They also offer different activities during the day which seem to be well attended. Many travelers travel to Mendoza on their own and take a few weeks of Spanish classes, so it's certainly a great place to meet up with other travelers. I'm personally studying to take the DELE--Diploma de Español como lengua extranjera and am getting some great private lessons.