1. I would change the font color of the results. Now the font has almost the same color as the background and it's really hard to read. This was my first impression when I have tested the tool.
2. If you are testing the PR of a longer domain the design brakes.
3. I would remove the yahoo backlink checker because this does not exist anymore.
4. I would add more features like: compete rank, tweets about the site, facebook likes, google plus, etc...
5. I would add an indexer, that create pages for each site the user tests, something that will look like this: ageRank & Site Analysis
This is all what I would change...Hope this helps!