Backpackers frequently stop by or email Hostal Tamarindo in Medellin offering to work as a volunteer in exchange for accommodation. This is easier said than done. Colombian laws only allow volunteering if the visa the person is travelling on is for the purpose of volunteering.
Upon arrival in Colombia, your passport gets stamped. The stamp has the number that corresponds to your type of visa and the number of days you may stay in the country before having to renew your visa. Most travellers travel on a PIP-5 visa, which is a tourist visa. This visa does NOT allow you to work as a volunteer.
If you want to volunteer, you must request a change to a TP-6 visa. Visit the following link for additional information on this type of visa.
You may also want to check out the law that prohibits volunteering as a tourist (It’s in Spanish, sorry!): Ley 720 de 2001
Having said all of this, you may still find hostels that are willing to hire volunteers and risk paying steep fines and or have their businesses shut down.