When I boot Ubuntu 10.04.3 from a live CD, it asks me to enter a username and password. I have not booted before, and I haven't created an account.
The default username is ubuntu, and the password is blank on an Ubuntu LiveCD.
Brent posted the following over on Serverault:
If the username "ubuntu" with an empty password doesn't work, you may be able to add a new user to the system as follows.
Press Ctrl+Alt+F1 and run this command:
sudo adduser username
Meanwhile in some other Ubuntu-Based Distros (especially official ones), the username goes by their distro name (e.g: Xubuntu='xubuntu', Lubuntu='lubuntu' etc) though you can also find the username of it on some distros by looking at the upper-left hand corner.
