Overall, I really like your idea of using whitespace to serve ads. I'd much rather offer those on my site than other types of ads. I think the over all concept works. If you study animation and motion desing, you realize that a viewer's eye is attracted by motion in an image. So, I think that it's a good idea, to have the slow fade in/fade out on the ads. It's not jarring, and abusive, I like that. Another strength is that the ad fades in beside the user's mouse cursor. So the user's eyes are probably looking at that part of the screen to begin with. Because of that, I suspect that you will continue to have better than average conversion rates. Congrats.

Constructive criticism:

Logo: From a design perspecive, it seems a little cramped between the "pt" in empty and the angle of the thought bubble. I'd like a little more space there, and perhapse angle the balloon a little bit, so it is lower below the "pt" and higher beside the "ds" in "spaceads". Just a thought. I think the logo is a good start, but it could use a little more work.

Company Name: In the design world, the term "white space" is used to describe what you're calling "empty space". If you're going after designers, that might be a plus. I don't know if you've done any domain name searches involving "white space", but at least to me, it would be a clearer explanation of what your company is doing than "empty space".

Front page design: Over all, I like the color scheme. However, the white on orange, "Sign up Now" and "Learn More," buttons are hard to read.

Front page copy: I had a couple of issues. The phrase, "Publishers turn your site's empty space into cash money" is a little problematic for me. The term "Publisher" communicates to me a magazine or book publisher, not a blogger or content creator. Those are your target demographic, I'd make it obvious I'm referring to them on my front page.

Also, the phrase, "cash money" is redundant and bugs me a bit. Copy like that is used in payday loan services or used car sales. It cheapens the service in my mind. I'd think that, "Bloggers. Change your site's blank space into cash." or something along those lines would be stronger copy.

Demo page: I didn't really like the quality of the ads that were served in the demo. It appears that you're using live ads from your ad network. I'd rather see something well designed rather than what comes in randomly off the ad network. I think that you'd get people's attention a bit easier. My first impression when I look at your site, is this: "Wow, looks pretty good." My first impression when I look at the demo ads is: "Huh??? What is that picture of? Oh, it's an ad for a hand bag... I think." The images that I saw on several re-loads were small and pixelated and frankly, I was turned off. I know that you might not have a lot of control over that, because you're only working with one ad affiliate. But for your demo, I'd make a different choice.

Demo page design: Just a simple thing, but my eye was attracted to the bright green in the pie chart first. That's not the most important part of your demo. It was a little distracting for me.

Again, I'm not trying to nit pick. I'm tryin to give what was asked for: an honest critique. Over all, I think that you're well on your way. Good job, and you're off to a great start. Keep it up.