As of last week, there was a 10% or 20% off "pop-up" ad that appeared when booking flights. There is even a direct link to the pop-up, but it doesn't always work even that way...
The direct link is:
http://www.flyi.com/specials/exitpopup.aspx
My recommendation would be to go through with the booking process the ENTIRE way until you get to payment... then hit the popup link. If you just try the pop-up link without spending some time on the site first, it will likely just give you a blank page.
You may need to clear cookies and/or cache before using the link, as well. Unless they've turned it off, it should still work; it was working as of last weekend.