UbuntuNight is a website of service platform co-established by Ubuntu fans from all over the world,which providing all available mirrors sorted by three Top-mirrors:
Top-fast is based on average download speed.
Top-near is by your geographical position(normally close to your country)
Top-stable is by download speed variance statics.

By now,the top-mirror only supports Ubuntu 10.04 LTS (Lucid Lynx).
You can use following commands in Applications->Accessories->terminal to:(It is recommended to backup your original source list file before overwriting.)

Choose Top fast Mirror:

sudo wget -O /etc/apt/sources.list http://ubuntu9.com/topmirror/sourceslist/topfast

Choose Top near Mirror:

sudo wget -O /etc/apt/sources.list http://ubuntu9.com/topmirror/sourceslist/topnear

Choose Top stable Mirror:

sudo wget -O /etc/apt/sources.list http://ubuntu9.com/topmirror/sourceslist/topstable