I have a very small inn - 3 rooms. I use Quickbooks Pro as a guest management system, of sorts, in conjuction with my ical - the Apple calender that came with the computer. I used to use a Palm calendar, but switched to the ical a while ago. Both calendars allow me to customize and color code everthing so I can see at a glance what is going on, and I can have all my personal appointments and birthdays on there too, so I don't have to keep more than one calendar. I think the google calendar can do this too, but I am not sure. I bet that PC's come with a similiar customizable calendar, don't they?
The Quickbooks part - Guest info is inputted after they depart, unless they need a computer receipt, which I will do upon check out. My rooms are set up as items, so pulling reports on them is easy. My reciepts are customized too.
The information you may enter on a guest is endless as the customer's page is cusomizable, as Proud TX has done. It IS a database of sorts.
The Customer List can be exported into a database program to create mailing lists. It can also be exported as a text file. You may do this with all the lists, item, vendor, whatever.
I have a very simple system because my inn is so small, but it is very easy to use. It doesn't come close to what the guest tracking software out there can do, but it is enough for me..