I consider Citrix Xen a great product with a lot of advantages compared to VMWare. However, one big disadvantage is the lack of a Virtual Appliance repo.
There are some sites that can provide you with appliances ready to work but they are very VMWare oriented. Example:
Although Jumpbox has some support for Citrix Xen and other virtualization platforms, I suggest to create a place where we can start to build our own Virtual appliances for Citrix Xen.
My first donation is the following.
Centos 5.3 64 bit with Xentools installed
You can get it from : ftp://ftp.carlosgomez.net/
you can try http://www.filebox.com/jlspdob2rn8k
Enjoy it ...