Channels to marketing an API-as-a-product to developers

My company provides a set of API for accessing telecommunication services (kind of like Twilio). The target customers are web developers. They could be indie developers, or companies.

What are some cost effective channels to reach out to developers?

I have tried Google Adwords, but the CPC is too high for acquiring developers. Any more cost effective ways?

• I first heard of Twilio from This Developer's Life Podcast. Podcasts
• Advertise directly on blogs / online magazines
• There are people who do training podcasts like railscasts.com - Ruby on Rails training. Find these people who have a strong following and see if they would do a video on your API/system. Could find leaders in Ruby, PHP, .NET, Python etc. Developers have favorite blogs, and there are also leaders / heroes in certain technologies
• Adwords, if it's an API for a new type of service then PPC shouldn't be too bad. If you are just trying to get any developers then maybe.
• Trade magazines / publications. Magazine ads are expensive but not ridiculous. They are always looking for new advertisers so you can often negotiate a trial placement or something
• Grassroots. Start with your local developer meetups and give away free coupons / promos
• Conferences. Speak at conferences and promote your product
• Give away subscriptions or coupons to prominent bloggers, or to meetups
• Sponsor startupweekend's or some meetup groups.

Good luck