Frontiers is an example of a collection of different open access journals where you find some high-impact publications within the cognitive science field. They have A LOT of journals within the neuroscience and the psychology field.
This is what Frontiers themselves says about impact factors of their journals:
Thomson ISI requires at least three complete years of publishing, plus a few of other criteria, before issuing an impact factor for a specific journal. In the case of Frontiers, this condition has so far been met by only three of our Neuroscience journals. Impact factors are issued for the preceding year involving complex criteria that evaluate the previous three years' citations.
The impact factor for Frontiers in Human Neuroscience was 1.58 in 2009
and 1.94 in 2010, indicating substantial growth year over year. We
expect to see this growth in Frontiers in Computational Neuroscience
(2.58 in 2010) and Frontiers in Cellular Neuroscience (3.58 in 2010),
as well as a number of impact factors issued to other Frontiers
journals in 2012. We expect these impact factors to continue to rise
in 2012 as well as to have Thomson ISI issue several more impact
factors to Frontiers journals in June 2012.
So, in this case "Frontiers in Human Neuroscience", "Frontiers in Computational Neuroscience" and "Frontiers in Cellular Neuroscience" would probably be three good answers to your question.