Like at least one other post I read here, if not for Coffeecup I would be lost. Keep up the good work!
Guest Book? Yes, I'd be interested in that. I would also like to see it have the ability to be backed up on a local machine. I tend to change hosts from time to time and lose what was entered. I have had more than one fold on me and I've had to srart over from the ground up. With close to 1000 pages (you read that right!) it is challenging at best.
Another thing I'd like to see in a Guest Book/Forum is the ability to use it on a Windows or Linux Server.
I have had more problems being spammed in Forums than Guest Books, but still the ability to manage a functions would be a must.