If you have young kids, Tiger's Parental Controls (in the Accounts preferences panel) offers choices like Simple Finder and pictures in list of users. The kids just click on their photo and then on the leftmost folder to get to their games and such.
Since a password is too difficult for most pre-schoolers to use, it's best to create these accounts without passwords. For anyone with access to the machine, this means they'll be able to login easily (but with Parental Controls enabled, they probably won't be able to do much harm).
But it is quite a comfort, then, that the Remote Login feature (i.e. ssh) is disabled by default for any no-password accounts. No one else can try to hack these simple accounts remotely...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050608163534540