I recommend Puppy Linux for a newbie's first distro.  My reasons:
1.  It's user-friendly.  As I found when I tried Debian, Puppy Linux is so user-friendly that it spoiled me into expecting other distros to be as easy to use.  My only gripe is that even the single-boot installation process requires manually editing the menu.lst file when all of the other distros I've tried took care of that automatically.  This is the one area where Puppy Linux could learn from antiX Linux.
2.  It installs in just a few minutes.  On the other hand, Fedora Core 1, Debian, and Ubuntu take a long time to install.
3.  It's lightweight - runs entirely from RAM on only 256 MB.  The only lighter distro I've used is Damn Small Linux.  I think Damn Small Linux skimps on many things in order to limit its size to 50 MB.  In DSL, I could only get Flash 7 to work, and that required a rather complex installation procedure.  Flash 9 was so unstable that the browser crashed within a few seconds.  I also found the printer installation process to be difficult and unintuitive.  I think 256 MB is marginal for Ubuntu.  Starting off with a command-line installation and adding packages allows for a more lightweight operation, but then Ubuntu isn't as user-friendly.  Ubuntu's hardware requirements keep escalating, and the earlier and lighter versions keep getting terminated, so a group of low-end users get cut off each year.  You can probably forget about running the latest version of Fedora with only 256 MB of RAM.
4.  If you don't need any specialized applications, Puppy Linux's small repository isn't an issue.  For the average person on the street (who just does web browsing, email, word processing, spreadsheets, and presentations), Puppy Linux is perfect.  That said, Puppy Linux is working on Debian compatibility.  I'd love to see a rivalry between Puppy Linux and antiX Linux, as the users of both as well as others using old computers would all win, and only Microsoft would lose.

-- 
Jason Hsu, Linux user <jhsu802701 at jasonhsu.com>