Nice one, it's certainly coming along - Give it a couple more months and I might start using this once you've got all the features sorted.
Please what ever you do with any new features retain the speed of the checker... Its nice and quick!
A few minor asthetic things:
1. Make the app maximisable (I know from experience that this isn't as easy as some people think becasue of layout etc but I'm not a big fan of this feature being disabled)
2. Use colour - when a domain is available highlight it as green and ready to be registered so that the availables stand out from the rest.
3. When you click on a name it doesn't highlight in blue... this would make it easier to read accross a line.
4. What is the mouse icon that appears when you hover over the names on the right?? Not needed in my opinion - It feels like theres something stuck to my mouse!
These are all user issues that will help anyone use your tool. Ive developed software for people before and user experience / ease of use is always important.
Functionality wise I'm sure you've got loads to add so I wont bother with any suggestions, you seem to be getting right on this project and good for you!!
Well done so far, its coming along very nicely!
Rich