Yes, I realize such a HowTo already exists (HowTo: enable passwordless logins via GDM), but this method is entirely different and, I think, less complicated.
Warning: Even though this method is simple (few steps), it is extremely dangerous if you don't know what you're doing. Do not attempt this if you are worried that you might mess up the /etc/shadow file, thus screwing up your Ubuntu system, possibly irreparably!
This is an oft-requested task, mainly for people who don't want to require less tech-savvy family members to have to remember passwords. It is a security risk, but I think people should at least know how to put their computers at risk if they want to. Don't blame me if anything bad happens.
Make sure you have created a user. For the sake of this example, let's say you called the account username guest. You can give it any temporary password you want. We're going to change that password shortly anyway. I'm assuming you know how to do this already. If you don't, I can assure you that this HowTo is not one you should be following, and you would be very likely to screw up the next step.
Next, go to the terminal and paste in this command:
This will open the /etc/shadow file (the one that contains all the passwords) in a text editor called Nano.
sudo nano -B /etc/shadow
Once you have it open, find the appropriate line for the account in question. It'll look something like this:
Change the second part (in bold here) to match this second part (also in bold):
Then save the file (Control-X, Y, Enter).
Now you should be able to log into the guest (or whatever you called it) account without entering a password.