I set up a xajax driven order procedure for our company.
What it does:
- Checks if a requested domain is free at the Whois database, on success displays the next part of the order procedure
- When user enters his own nameservers checks if these nameservers have the right syntax and if they exist
- Calculates all fees
- Checks on the validity of tax numbers
- Processes the whole thing to Paypal.
All in the background. Hooooray.

I was very excited that I was able to do things with xajax that are not only *funky* but actually very useful.
This process saves the users a ot of frustration like filling in a long form just to find out that a domain name was taken or there was a typo somewhere....
If you want to see it it's here Domain order form