I work for an established company - we are only 60ish people, but the company has been around for a very long time now. The resumes I see often contain companies I have never heard of and that is perfectly fine - as long as you can tell me what cool things you worked on, you would be a candidate I would listen to. Just because you haven't worked at some larger agency, it really wouldn't matter. It's how good of a coder that you are that really matters.