I'll tag on to Nelson's post to plug a new geocoding service two of us ex-Mapzen folks started called https://geocode.earth.

We offer geocoding, especially autocomplete, with much more liberal usage restrictions than either Mapquest or Google, and some pretty powerful features like address interpolation.

As we're still really getting the platform around the geocoder off the ground, we don't have a permanent free tier, but I'd be happy to set anyone up with a demo key if they are looking for something new.