What are some low cost marketing channels for startups?

One of the most effective channel is the site you are on right now - Quora. It has an advantage that unlike most of the social networking site, people actually come here to read. So if your product is really good, it’ll catch attention.
You have to be very specific of your target audience. Let’s say that the startup is about movie reviews and television shows, then they may advertise in torrent sites (Kattorent, Torrentz.com) That is pretty cheap compared to other advertising media.
Post media content on social networking sites. Videos are most effective.
Make a list of tech bloggers with good viewership and ask them to write a blog on your startup. By all means, it’ll reach out to a good number of people. You can also start your own blogs. There are apps that help you with the vocabulary likeVoLT - Vocabulary Learning - Android Apps on Google Play
Use Facebook’s sponsored stories. That ought to help you out.